Gestational Age Calculator

Calculate your gestational age, due date, and track pregnancy progress using four different methods: LMP, conception date, ultrasound, or IVF transfer.

Calculation Method

First Day of Last Menstrual Period (LMP) The first day of your last period before pregnancy

Gestational Age —

Due Date —

Trimester —

Days Until Due Date —

Your Baby This Week

How to Use the Gestational Age Calculator

Choose your method: Select LMP (most common), conception date, ultrasound dating, or IVF transfer depending on what information you have.
Enter the date: Input the relevant date and any additional information required for your chosen method.
View results instantly: See your current gestational age, estimated due date, trimester, and week-by-week development information.
Track your pregnancy: Bookmark this page and return weekly to see updated information about your baby's development.

Understanding Gestational Age

Gestational age is the number of weeks and days since the first day of your last menstrual period (LMP). This is the standard measurement used throughout pregnancy by obstetricians, midwives, and healthcare systems worldwide. Importantly, gestational age is approximately two weeks longer than fetal age (the actual time since conception) because it includes the two weeks before ovulation and conception occurred.

A full-term pregnancy is 40 weeks (280 days) of gestational age, calculated from the LMP. However, only about 4% of babies arrive exactly on their due date. The "normal" range for delivery is 37-42 weeks, with most babies born between 38-41 weeks. Babies born before 37 weeks are considered preterm (premature), while those born after 42 weeks are considered post-term.

Understanding gestational age is crucial because it determines prenatal testing schedules (genetic screening, anatomy scans, glucose testing), monitoring frequency, and delivery planning. Different pregnancy milestones and potential complications are associated with specific gestational ages. For example, the anatomy ultrasound is performed at 18-22 weeks, glucose screening at 24-28 weeks, and Group B strep testing at 35-37 weeks. Accurate gestational age dating ensures these tests happen at the optimal times.

The concept of gestational age can be confusing because you're not actually pregnant during the first two weeks of "pregnancy" — conception typically happens around week 2-3 of gestational age. This dating method was established because the LMP is a concrete, known date for most women, while the exact conception date is usually uncertain (sperm can survive 5 days in the female reproductive tract, and ovulation timing varies).

Four Methods to Calculate Gestational Age

1. LMP Method (Last Menstrual Period) — Most Common

Best for: Women with regular 28-day cycles who remember their LMP date.

How it works: Gestational age is calculated from the first day of your last menstrual period. The due date is calculated using Naegele's Rule: LMP + 280 days (40 weeks), or equivalently, LMP + 7 days - 3 months + 1 year.

Formula: Gestational Age = Today's Date - First Day of LMP
Due Date = LMP + 280 days

Example: If your LMP started January 1, 2026, and today is March 1, 2026 (59 days later): Gestational age = 8 weeks, 3 days. Due date = October 8, 2026.

Accuracy: This method works well for women with regular 28-30 day cycles. It's less accurate for irregular cycles, longer cycles (35+ days), or if you conceived while breastfeeding or after stopping birth control. In these cases, ultrasound dating is recommended to confirm or adjust the due date.

Limitations: Assumes ovulation occurred on day 14 of your cycle, which isn't true for all women. Women with PCOS, irregular cycles, or those who conceived immediately after birth control may ovulate much earlier or later. The LMP method can be off by 1-2 weeks in these situations.

2. Conception/Ovulation Date Method

Best for: Women who tracked ovulation with tests, BBT charting, or know exactly when they conceived.

How it works: If you know when you ovulated or conceived, add 2 weeks (14 days) to get gestational age. This accounts for the 2-week period between LMP and conception in a typical 28-day cycle.

Formula: Gestational Age = (Today's Date - Conception Date) + 2 weeks
Due Date = Conception Date + 266 days (38 weeks from conception = 40 weeks from LMP)

Example: If you conceived on January 15, 2026, and today is March 1, 2026 (45 days later): Gestational age = 45 days + 14 days = 59 days = 8 weeks, 3 days. Due date = October 8, 2026 (266 days after conception).

Accuracy: More accurate than LMP method if you genuinely know your ovulation/conception date. Women using fertility awareness methods, ovulation predictor kits (OPKs), or fertility tracking apps often have this information.

Note: If you're using conception method, make sure you're using the actual conception date (when the egg was fertilized), not just when you had intercourse. Sperm can survive up to 5 days, so conception could occur days after intercourse.

3. Ultrasound Dating Method — Most Accurate

Best for: Everyone, especially women with irregular cycles, uncertain LMP, or when LMP dating seems inconsistent.

How it works: Early ultrasounds (before 13 weeks) measure the embryo/fetus and use standardized growth charts to determine gestational age with high accuracy. The crown-rump length (CRL) measurement in the first trimester is the most accurate dating method, with precision of ±3-5 days.

When performed: Dating ultrasounds are typically done at 8-13 weeks. Earlier ultrasounds (6-8 weeks) may be done to confirm pregnancy but are slightly less accurate for dating. Later ultrasounds (after 13 weeks) measure multiple parameters (biparietal diameter, head circumference, abdominal circumference, femur length) but are less accurate for dating because fetal growth becomes more variable.

Formula: If your ultrasound at a known date showed a specific gestational age, work backwards:
Due Date = Ultrasound Date + (280 days - gestational age at ultrasound in days)

Example: Ultrasound on February 1, 2026 showed 8 weeks, 3 days (59 days). Due date = February 1 + (280 - 59) = February 1 + 221 days = September 10, 2026. Today's gestational age = (Today - Ultrasound Date) + 59 days.

Accuracy: First trimester ultrasound dating is accurate within 3-5 days. This is more accurate than LMP dating, especially for women with irregular cycles. If there's a discrepancy of more than 7 days between LMP dating and ultrasound dating, most doctors will use the ultrasound date as the official due date.

Important: Once your due date is established by early ultrasound, it should not be changed by later ultrasounds. Second and third trimester ultrasounds have margins of error of ±1-2 weeks because babies grow at different rates. If a later ultrasound suggests a different gestational age, doctors will note it but rarely change the official due date unless there's strong evidence of growth restriction or other issues.

4. IVF (In Vitro Fertilization) Method — Most Precise

Best for: Women who conceived through IVF, ICSI, or frozen embryo transfer.

How it works: IVF dating is the most precise method because the exact fertilization date and embryo age are known. The gestational age is calculated based on the embryo transfer date and the age of the embryo at transfer (Day 3 or Day 5/6 typically).

Formula:
- For Day 3 transfer: Gestational Age = (Today - Transfer Date) + 2 weeks + 3 days
- For Day 5 transfer: Gestational Age = (Today - Transfer Date) + 2 weeks + 5 days
- For Day 6 transfer: Gestational Age = (Today - Transfer Date) + 2 weeks + 6 days

Example: Day 5 blastocyst transfer on January 1, 2026. Today is March 1, 2026 (59 days later). Gestational age = 59 days + 14 days + 5 days = 78 days = 11 weeks, 1 day. Due date = Transfer Date + (280 - 19) days = September 18, 2026.

Why this works: A Day 3 embryo is 3 days past fertilization. In a natural cycle, this would be approximately 17 days from LMP (14 days until ovulation + 3 days). A Day 5 blastocyst is 5 days post-fertilization, equivalent to 19 days from LMP. The calculation adds these days to account for the standard 2-week period before conception.

Accuracy: IVF dating is extremely accurate because the fertilization date is known precisely. There's no uncertainty about when conception occurred. IVF babies' gestational ages are considered "definitive" and are rarely adjusted even if ultrasound measurements differ slightly. This is especially important for IVF pregnancies because many IVF patients have irregular cycles or PCOS, making LMP dating unreliable.

Note for frozen embryo transfer (FET): The calculation is the same regardless of whether the embryo was fresh or frozen. What matters is the embryo age at transfer (Day 3, Day 5, etc.), not how long it was frozen.

Pregnancy Trimesters and Key Milestones

Pregnancy is divided into three trimesters, each roughly 13 weeks long. Each trimester brings distinct developmental milestones, physical changes, and prenatal care requirements.

First Trimester (Weeks 1-13)

Key developments: This is the period of the most dramatic development. In just 13 weeks, a single fertilized cell divides and differentiates into all major organ systems. By the end of the first trimester, your baby has a beating heart (detectable at 6 weeks), developing brain, forming limbs, fingers, and toes, and all major organs present (though not fully mature). The embryo becomes a fetus at 10 weeks. Gender is determined at conception but isn't visible on ultrasound until around 16-20 weeks.

Size progression: Week 4: poppy seed (2mm). Week 6: lentil (5mm), heart beats. Week 8: raspberry (16mm), arms and legs forming. Week 10: strawberry (31mm), officially a fetus. Week 12: plum (54mm), all organs formed.

Maternal symptoms: Morning sickness (nausea/vomiting, affects 70-80% of women, usually peaks at 9-10 weeks and improves by 14-16 weeks), extreme fatigue (from progesterone and increased metabolic demands), breast tenderness and swelling, frequent urination (from increased blood volume and uterine pressure), mood swings (hormonal fluctuations), food aversions and cravings, constipation, mild cramping. Many women don't "show" yet as the uterus is still in the pelvis.

Prenatal care: First prenatal appointment (8-10 weeks) includes confirmation of pregnancy, dating ultrasound, prenatal blood work (blood type, Rh factor, anemia screening, immunity testing for rubella and varicella, hepatitis B, HIV, syphilis). First trimester screening (11-14 weeks) includes nuchal translucency ultrasound and blood tests (PAPP-A, hCG) to screen for chromosomal abnormalities like Down syndrome (Trisomy 21). NIPT (non-invasive prenatal testing) may be offered 10+ weeks for more detailed chromosomal screening via maternal blood test.

Miscarriage risk: Highest in first trimester, especially before 12 weeks. About 10-20% of known pregnancies end in miscarriage, with 80% occurring in the first trimester. Risk drops significantly after detecting a heartbeat at 8 weeks (decreases to about 2-5%). Most first trimester miscarriages are due to chromosomal abnormalities incompatible with life, not maternal behavior.

Second Trimester (Weeks 14-27)

Key developments: Rapid growth period. Baby's skin becomes less transparent, hair begins growing (lanugo - fine body hair, and scalp hair), fingerprints develop, baby practices breathing movements, swallowing amniotic fluid, and making facial expressions. Senses develop: hearing (responds to sounds at 18 weeks), taste, touch. Fat begins accumulating under skin. By 24 weeks, babies have a chance of survival if born prematurely (with intensive NICU care).

Size progression: Week 14: lemon (87mm). Week 16: avocado (116mm), gender potentially visible on ultrasound. Week 20: banana (260mm), anatomy scan performed. Week 24: ear of corn (300mm), viability milestone. Week 27: cauliflower (365mm).

Maternal symptoms: "Honeymoon trimester" — most women feel much better. Morning sickness usually subsides by 14-16 weeks. Energy returns. Visible baby bump emerges (typically 16-20 weeks for first pregnancies, earlier for subsequent). Quickening (feeling baby move) occurs 16-22 weeks (earlier for experienced mothers, later for first-time moms). Round ligament pain (sharp pains in lower abdomen from stretching ligaments). Nasal congestion (pregnancy rhinitis). Skin changes (linea nigra dark line on belly, darkened areolas, possible melasma on face). Braxton Hicks contractions may begin (practice contractions, irregular and painless).

Prenatal care: Anatomy scan (18-22 weeks, typically week 20) — detailed ultrasound examining all fetal organs, limbs, placenta position, amniotic fluid, and growth. Gender reveal if desired. Glucose screening (24-28 weeks) for gestational diabetes (50g glucose challenge test; if positive, followed by 3-hour glucose tolerance test). Regular prenatal visits every 4 weeks. Fetal movement counting becomes important.

Milestones: Week 20 — halfway point! Many women celebrate with maternity photos or gender reveal. Week 24 — viability (babies born at 24 weeks have about 60-70% survival with NICU care, though risk of complications is high).

Third Trimester (Weeks 28-40+)

Key developments: Final maturation and dramatic weight gain. Baby's lungs mature (surfactant production begins around 24 weeks, peaks at 35 weeks). Brain development accelerates. Fat accumulates, making skin less wrinkled. Baby settles into birth position (ideally head-down by 36 weeks). Bones harden (except skull bones, which remain soft for passage through birth canal). Baby's sleep-wake cycles become more regular. By 37 weeks, baby is considered "early term" and organs are mature enough for life outside the womb without significant complications.

Size progression: Week 28: large eggplant (375mm). Week 32: jicama (425mm), lungs nearly mature. Week 36: papaya (475mm), considered "late preterm" if born. Week 37-40: watermelon (480-520mm), full term. Week 40+: pumpkin (510mm+), post-dates.

Maternal symptoms: Growing discomfort as baby and uterus expand dramatically. Shortness of breath (uterus pushes up on diaphragm), heartburn and indigestion (progesterone relaxes esophageal sphincter; uterus pressure), frequent urination returns (baby's head presses on bladder), pelvic pressure and discomfort, swollen feet and ankles (fluid retention), difficulty sleeping (no comfortable position, frequent bathroom trips, baby movement), back pain, Braxton Hicks contractions increase in frequency and intensity. Possible "nesting instinct" (strong urge to clean and organize). Emotional fluctuations about impending labor and parenthood.

Prenatal care: Visits increase in frequency: every 2 weeks from 28-36 weeks, then weekly from 36 weeks until delivery. Group B strep (GBS) screening (vaginal/rectal swab at 35-37 weeks) — if positive, IV antibiotics during labor prevent neonatal infection. Growth monitoring (measuring fundal height to track baby's growth; ultrasound if concerns). Fetal position assessment (ensuring baby is head-down; if breech at 36+ weeks, external cephalic version may be attempted). Non-stress tests (NST) may begin at 40-41 weeks to monitor baby's well-being. Biophysical profile (ultrasound + NST) if pregnancy goes beyond 40-41 weeks.

Term definitions: Early term (37-38 weeks, 6 days): organs mature but babies have slightly higher risks vs full term. Full term (39-40 weeks, 6 days): optimal time for delivery; best outcomes. Late term (41 weeks to 41 weeks, 6 days): increased monitoring; induction often discussed. Post-term (42+ weeks): significantly increased risks (placental aging, low amniotic fluid, stillbirth); induction usually recommended by 41-42 weeks.

People Also Search For

How to calculate due date from last menstrual period

To calculate due date from last menstrual period (LMP), add 280 days (40 weeks) to the first day of your last period, or use Naegele's Rule: add 1 year, subtract 3 months, add 7 days. For example, if your LMP was January 1, 2026: add 1 year = January 1, 2027; subtract 3 months = October 1, 2026; add 7 days = October 8, 2026 (your due date). The calculator above does this automatically and also shows your current gestational age, trimester, and expected delivery date range. Remember that only 5% of babies arrive on their exact due date.

What is the difference between gestational age and fetal age

Gestational age is calculated from the first day of your last menstrual period (LMP) and is ~2 weeks longer than fetal age, which is calculated from conception/fertilization. Gestational age is the standard used by doctors and includes the ~2 weeks before conception when your body was preparing to ovulate. Fetal age (also called embryonic age or conceptional age) starts at fertilization. If you're 10 weeks gestational age, the fetus is approximately 8 weeks old. Medical professionals always use gestational age for consistency.

How many weeks am I pregnant calculator

To find how many weeks pregnant you are, enter the first day of your last menstrual period in the calculator above, and it instantly shows your gestational age in weeks and days. The calculator also shows which trimester you're in, your due date, and the date range when delivery is most likely (37-42 weeks). If you don't remember your LMP, use the "Conception Date" method and enter when you likely conceived (typically 14 days after LMP for regular cycles). First trimester ultrasound can also date the pregnancy with ±3-5 days accuracy.

When will I have my baby based on conception date

To calculate your due date from conception date, add 266 days (38 weeks) to the conception date. For example, if conception was February 1, 2026, your due date is approximately October 25, 2026. Use the calculator above by selecting "Conception Date Method" and entering when you conceived. IVF pregnancies use the exact fertilization date, making this the most accurate dating method. Natural conception typically occurs around ovulation, which is ~14 days after the start of your last period for women with regular 28-day cycles.

What trimester am I in pregnancy calculator

Use the calculator above to instantly see which trimester you're in based on gestational age: First trimester (weeks 1-13), Second trimester (weeks 14-27), Third trimester (weeks 28-40+). Each trimester has distinct developments and maternal symptoms. First trimester focuses on organ formation, second trimester is the "golden period" with less nausea and more energy, and third trimester involves final maturation and weight gain. The calculator shows your current week, trimester, and due date all at once.

Is due date based on 40 weeks or 9 months

Due date is based on 40 weeks (280 days) from the first day of your last menstrual period, which equals approximately 9 months and 1 week. The confusion arises because pregnancy is ~9 calendar months but 10 lunar months (4-week months). Medical professionals always use weeks, not months, for precision. A full-term pregnancy is 37-42 weeks, with 40 weeks being the estimated due date. Only about 5% of babies arrive exactly on their due date—the "due date" is really an estimated delivery window of about 5 weeks (37-42 weeks).

Frequently Asked Questions

Which calculation method is most accurate?

First trimester ultrasound (before 13 weeks) is the most accurate method for spontaneous pregnancies, with accuracy of ±3-5 days. IVF dating is even more precise because the exact fertilization date is known. LMP method is less accurate, especially for women with irregular cycles, but it's still the starting point for dating and works well for women with regular 28-30 day cycles. If you have irregular cycles or uncertain LMP, request an early dating ultrasound.

Can I use this calculator if I have irregular periods?

If you have irregular periods, the LMP method will be inaccurate because it assumes ovulation occurred on day 14. You should either use the conception date method (if you tracked ovulation with OPKs or BBT charting) or, ideally, get an early dating ultrasound (6-13 weeks) for accurate gestational age. Tell your healthcare provider about irregular cycles at your first prenatal visit so they can arrange appropriate dating.

What if I don't remember my last menstrual period?

If you don't remember your LMP, you'll need an ultrasound to date the pregnancy. Schedule a prenatal appointment as soon as possible. Your doctor will perform a dating ultrasound (ideally before 13 weeks) to establish gestational age based on embryo/fetal measurements. This is also common for women who conceived while breastfeeding (periods may not have returned) or immediately after stopping hormonal birth control.

Why am I 2 weeks pregnant but haven't conceived yet?

This confusing aspect of gestational age dating means the "2 week pregnant" mark is actually the first day of your LMP — before ovulation and conception even occurred. You're not actually pregnant yet at 0-2 weeks gestational age. Conception typically happens around 2-3 weeks (at ovulation). The embryo implants around 3-4 weeks. Most women discover they're pregnant around 4-5 weeks (when the missed period occurs). Despite being "4 weeks pregnant," the embryo has only existed for about 2 weeks since conception.

When is the best time to get a dating ultrasound?

The most accurate dating ultrasounds are performed between 8-13 weeks gestational age. At this stage, the crown-rump length (CRL) measurement correlates very precisely with gestational age. Ultrasounds before 6-7 weeks may not show a heartbeat yet (which can cause unnecessary anxiety), while ultrasounds after 13 weeks are less accurate for dating because fetal growth becomes more variable based on genetics and nutrition. If you need dating clarification, aim for an 8-12 week ultrasound.

What is Naegele's Rule?

Naegele's Rule is the traditional formula for calculating due date from LMP: Take the first day of your last menstrual period, add 7 days, subtract 3 months, and add 1 year. For example, LMP = April 10, 2025. Add 7 days = April 17. Subtract 3 months = January 17. Add 1 year = January 17, 2026 (due date). Alternatively, just add 280 days (40 weeks) to the first day of LMP. This rule assumes a 28-day cycle with ovulation on day 14 and works reasonably well for most women with regular cycles.

How does IVF dating differ from natural conception dating?

IVF dating is more precise because the fertilization date is known exactly. For IVF pregnancies, gestational age is calculated based on embryo transfer date plus the embryo age at transfer, adjusted to match gestational age convention (adding 2 weeks pre-conception time). A Day 5 blastocyst transfer on January 1 means conception occurred December 27 (5 days before transfer), and gestational age dating starts December 13 (2 weeks before conception). This precision is especially helpful for IVF patients who often have irregular cycles or PCOS, making LMP dating impossible.

Can the due date be wrong?

The due date is an estimate, not a guarantee. Even with perfect dating via first trimester ultrasound or IVF, only 4-5% of babies arrive exactly on the due date. About 90% of babies are born between 37-42 weeks (3 weeks before to 2 weeks after the due date). Factors affecting timing include genetics (mothers who delivered early/late tend to follow similar patterns), first vs subsequent pregnancy (first babies average 8 days past due date, subsequent babies average 3 days past), maternal age, baby's sex (boys slightly more likely to be late), and placental function.

What happens if I go past my due date?

Going past your due date is normal — about 50% of first-time mothers deliver after 40 weeks. Most doctors will allow pregnancy to continue naturally until 41-42 weeks with increased monitoring (NSTs, ultrasounds to check amniotic fluid and placental function). The risks of going past 42 weeks increase significantly (placental aging, decreased amniotic fluid, meconium in fluid, stillbirth risk rises from 1 in 1000 at 40 weeks to 3 in 1000 at 42 weeks), so most doctors recommend induction between 41-42 weeks if labor hasn't started naturally.

When will I feel my baby move?

First-time mothers typically feel baby movement (quickening) between 18-22 weeks, averaging around 20 weeks. Women who've been pregnant before usually feel movement earlier, around 16-18 weeks, because they recognize the sensation. The baby is actually moving much earlier (from 8 weeks), but the movements are too small to feel. Initially, movement feels like butterfly flutters, bubbles, or gas. By 24-28 weeks, movements become stronger kicks and rolls that are unmistakable. Anterior placenta (placenta on the front uterine wall) can muffle movement, causing women to feel it later.

What is considered full term?

Full term is defined as 39 weeks 0 days through 40 weeks 6 days. The American College of Obstetricians and Gynecologists (ACOG) refined term definitions in 2013: Early term = 37-38 weeks 6 days (organs mature but slightly higher risks), Full term = 39-40 weeks 6 days (optimal timing for best outcomes), Late term = 41 weeks 0 days through 41 weeks 6 days (increased monitoring recommended), Post-term = 42 weeks 0 days and beyond (significantly increased risks, induction typically recommended). This replaced the old "term = 37-42 weeks" definition because outcomes are best at 39-40 weeks.

How often should I have prenatal visits?

Standard prenatal visit schedule for low-risk pregnancies: Every 4 weeks from first appointment (8-10 weeks) through 28 weeks. Every 2 weeks from 28-36 weeks. Weekly from 36 weeks until delivery. High-risk pregnancies require more frequent visits. Each visit includes weight, blood pressure, urine dipstick, fundal height measurement, fetal heart rate check, and discussion of symptoms. Major appointments: First visit (8-10 weeks) for confirmation and baseline labs. 11-14 weeks for first trimester screening. 18-22 weeks for anatomy ultrasound. 24-28 weeks for glucose screening. 35-37 weeks for Group B strep testing.

Can stress or exercise change my due date?

No, the due date is determined by when conception occurred, which cannot be changed. Stress, exercise, diet, and activity levels during pregnancy affect maternal health and potentially baby's growth, but they don't change when conception happened or when the 40-week mark will be reached. However, some research suggests that stress, nutrition, and activity may influence when labor begins naturally within the normal term range (37-42 weeks). For example, moderate exercise throughout pregnancy may slightly reduce the chance of post-term pregnancy, but more research is needed.

What if my baby measures small or large on ultrasound?

After gestational age is established via early ultrasound, later ultrasounds assess growth rather than redate the pregnancy. If baby measures small (below 10th percentile for gestational age), doctors evaluate for intrauterine growth restriction (IUGR) due to placental problems, maternal health issues, or genetic factors. If baby measures large (above 90th percentile), doctors consider gestational diabetes, maternal diabetes, or genetics (tall parents = bigger babies). Serial ultrasounds and additional monitoring may be needed, but the official due date doesn't change — instead, doctors note size discrepancy and adjust management accordingly.

Is it safe to exercise during pregnancy?

For most women with uncomplicated pregnancies, exercise is safe and beneficial. ACOG recommends at least 150 minutes of moderate-intensity aerobic activity per week throughout pregnancy. Safe activities: walking, swimming, stationary cycling, prenatal yoga, light strength training. Avoid: contact sports, activities with fall risk (skiing, horseback riding), hot yoga/hot tubs (overheating risk), scuba diving, and lying flat on your back after 20 weeks (supine hypotension). Listen to your body, stay hydrated, avoid overheating, and stop if you experience vaginal bleeding, contractions, dizziness, chest pain, or decreased fetal movement. Always discuss exercise plans with your healthcare provider, especially if you have complications like placenta previa, preterm labor risk, or pregnancy-induced hypertension.

Last reviewed: February 2026 — formulas and guidelines verified.