Pregnancy Week by Week Calculator
Track your pregnancy progress with detailed week-by-week information on fetal development, trimester, and due date.
What Is a Pregnancy Week by Week Calculator?
A pregnancy week by week calculator is a comprehensive tool that tracks your pregnancy progress from conception to birth. It provides detailed information about fetal development, mother's body changes, important milestones, and what to expect each week of your 40-week pregnancy journey.
This calculator helps expectant parents:
- Track fetal development: Learn how your baby is growing each week
- Know your trimester: Understand which pregnancy stage you're in
- Plan appointments: Know when to schedule prenatal visits and tests
- Understand symptoms: Learn what's normal for each week
- Prepare for birth: See exactly how many weeks until your due date
How Pregnancy Weeks Are Calculated
Pregnancy is typically calculated from the first day of your last menstrual period (LMP), even though conception usually occurs about 2 weeks later. This is because:
- Standard medical practice: The LMP method is universally used by healthcare providers
- Easy to track: Most women remember their last period date
- Conception date uncertainty: The exact conception date is often unknown
- 40-week standard: Full-term pregnancy is 40 weeks from LMP (280 days)
Formula: Gestational age (weeks) = (Today's date - LMP date) ÷ 7
Due date formula: LMP date + 280 days = Expected due date (Naegele's Rule)
Pregnancy Trimesters Explained
Pregnancy is divided into three trimesters, each with distinct characteristics:
First Trimester (Weeks 1-13)
- Key developments: All major organs form, heart begins beating
- Mother's symptoms: Morning sickness, fatigue, breast tenderness
- Important milestones: First prenatal visit, first ultrasound (8-12 weeks)
- Size: By week 13, baby is about 3 inches long
Second Trimester (Weeks 14-27)
- Key developments: Baby's movements felt, sex can be determined
- Mother's symptoms: "Honeymoon phase" - more energy, less nausea
- Important milestones: Anatomy scan (18-22 weeks), glucose screening (24-28 weeks)
- Size: By week 27, baby is about 14 inches long, 2 lbs
Third Trimester (Weeks 28-40)
- Key developments: Rapid growth, lungs mature, baby positions for birth
- Mother's symptoms: Back pain, frequent urination, Braxton Hicks contractions
- Important milestones: Weekly checkups (36+ weeks), baby may drop into pelvis
- Size: Full-term baby averages 19-21 inches, 6-9 lbs
Baby Size Comparison by Week
Medical professionals often compare fetal size to familiar fruits and vegetables:
- Week 8: Raspberry (½ inch)
- Week 12: Lime (2 inches)
- Week 16: Avocado (4.5 inches)
- Week 20: Banana (10 inches)
- Week 24: Cantaloupe (12 inches)
- Week 28: Eggplant (14 inches)
- Week 32: Squash (16 inches)
- Week 36: Honeydew melon (18 inches)
- Week 40: Small pumpkin (19-21 inches)
Key Fetal Development Milestones
- Week 5-6: Heart begins beating (detectable by ultrasound)
- Week 8: All major organs have formed
- Week 10: Officially called a "fetus" (no longer an embryo)
- Week 12: Reflexes develop, can suck thumb
- Week 16: Sex organs visible on ultrasound
- Week 18-22: Mother feels movement ("quickening")
- Week 24: Viability threshold (survival possible with medical care)
- Week 28: Eyes can open, responds to sounds
- Week 32: Lungs nearly mature
- Week 37: Full-term, ready for birth
Prenatal Appointment Schedule
Standard prenatal care schedule:
- Weeks 4-28: Every 4 weeks (monthly)
- Weeks 28-36: Every 2 weeks (bi-weekly)
- Weeks 36-40: Every week (weekly)
- Past 40 weeks: Twice weekly until delivery
Important tests and screenings:
- 8-12 weeks: First ultrasound, dating scan
- 10-13 weeks: First trimester screening (optional)
- 15-20 weeks: Maternal serum screening (quad screen)
- 18-22 weeks: Anatomy scan (detailed ultrasound)
- 24-28 weeks: Glucose tolerance test (gestational diabetes screening)
- 35-37 weeks: Group B strep test
How Accurate Are Due Dates?
Only about 4% of babies are born on their exact due date. Here's the reality:
- Full-term range: 37-42 weeks is considered normal
- 50% of first-time mothers: Give birth after their due date
- Average first pregnancy: 40 weeks + 5 days
- Average subsequent pregnancies: 40 weeks + 3 days
- Induction: Usually offered if you reach 41-42 weeks without labor
Your due date is best viewed as an estimate – a "due month" rather than a specific date.
Early Pregnancy vs Late Pregnancy
Early Pregnancy Signs (Weeks 1-12)
- Missed period (most obvious sign)
- Morning sickness (nausea, vomiting)
- Tender, swollen breasts
- Fatigue and exhaustion
- Frequent urination
- Food cravings or aversions
Late Pregnancy Signs (Weeks 28-40)
- Braxton Hicks contractions (practice contractions)
- Shortness of breath
- Heartburn and indigestion
- Swelling in feet, ankles, hands
- Lower back pain
- Baby "dropping" (lightening) near the end
People Also Search For
What week of pregnancy am I in?
Enter your last menstrual period (LMP) date in the calculator above and it will show your current gestational week instantly. Gestational age is counted from the first day of your last period, so the calculator computes the number of weeks and days that have elapsed since then. You'll also see your trimester, due date, and a progress bar showing how far along your pregnancy is.
How far along am I in my pregnancy?
Gestational age is counted from your LMP date, and at 40 weeks you are considered at term — the calculator shows your exact week, day count, and trimester. Being "8 weeks pregnant" means 8 weeks have passed since your last period, even though actual conception occurred around week 2. The calculator also displays days until your due date and a completion percentage so you can visualize your progress.
Pregnancy tracker by due date?
If you already know your due date but not your LMP, use the "Due Date" tab in the calculator — it counts back 40 weeks (280 days) to derive your LMP and then calculates your current week. This is handy when your due date was set by an early ultrasound rather than your last period. Enter the due date your doctor provided and the calculator will tell you your gestational age, trimester, and days remaining.
Baby development week by week?
Each week of pregnancy has specific developmental milestones, and the calculator shows key developments for your current week. In the first trimester all major organs form; by week 10 the embryo becomes a fetus; by week 16 sex organs are visible on ultrasound; by week 24 the baby reaches viability. The results include a size comparison to a familiar fruit or vegetable so you can visualize how your baby is growing right now.
What trimester am I in?
The first trimester covers weeks 1–13, the second trimester covers weeks 14–27, and the third trimester spans weeks 28–40; the calculator displays your trimester automatically alongside your gestational week. Each trimester brings distinct symptoms, developmental milestones, and prenatal appointments. First trimester focuses on organ formation; second trimester is often called the "honeymoon phase" with more energy; third trimester is rapid growth toward birth readiness.
How accurate is a pregnancy week calculator?
This calculator is based on ACOG Naegele's Rule (LMP + 280 days), the same standard used by OB-GYNs worldwide, and its accuracy depends entirely on knowing your correct LMP date. If your cycles are irregular or you are unsure of your LMP, ultrasound dating before 20 weeks is the most precise method and will give you a more reliable due date. Once you have an ultrasound-confirmed due date, you can enter it in the "Due Date" tab for equally accurate results going forward.