Free Pregnancy Week-by-Week Calculator

Find your current pregnancy week, trimester, and days remaining. See this week's developmental milestone, baby's size comparison, and upcoming milestones.

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What is this calculator for?

You're 17 weeks pregnant, you check your pregnancy app every Tuesday morning to see what your baby is the size of this week, and you want a simple tracker that shows current week, what's developing, what symptoms to expect, and when the next appointment falls. The pregnancy week calculator gives you that snapshot — where you are in the 40-week journey and what's happening this week.

Pregnancy is measured in completed weeks from the first day of your last menstrual period (LMP). Full term is 37-42 weeks. Most clinical milestones, appointment schedules, and fetal development markers reference specific weeks: 12-week ultrasound, 20-week anatomy scan, 28-week glucose test, 36-week group B strep test. Knowing your current week orients you in the timeline and helps you ask the right questions at each appointment.

This calculator takes your LMP (or due date) and returns your current week, trimester, days remaining, baby's developmental highlights this week, common symptoms this week, and upcoming appointments. Use it weekly for orientation; consult your OB for specific medical questions.

How to use this calculator

Enter your last menstrual period date (first day of your last period before conception) — the standard medical reference point. Alternatively, enter your due date if known; the calculator will derive LMP backward (due date − 280 days).

The calculator computes today's pregnancy week from LMP. Example: LMP March 15, today is May 12 → 8 weeks 3 days pregnant.

The output includes a week-specific developmental summary: what's happening with the baby (organs forming, movements becoming felt, lungs maturing, etc.), what's happening with your body (uterine growth, common symptoms), and clinical milestones (appointments, tests, vaccinations).

Understanding your results

The calculator returns your current week, trimester, days until due date, and a developmental snapshot for this specific week.

Pregnancy trimester boundaries: First (weeks 1-13), Second (weeks 14-27), Third (weeks 28-40+). Most women find Trimester 1 hardest (morning sickness, fatigue, miscarriage worry). Trimester 2 is the "honeymoon" trimester (energy returns, nausea fades, baby movements start). Trimester 3 brings the home stretch — physical discomfort returns, fetal movement is strong, planning for delivery dominates.

Key developmental highlights by week range:

Weeks 1-4: Implantation, neural tube formation. Most women don't yet know they're pregnant.

Weeks 5-8: Heart starts beating (week 5-6 — detectable on ultrasound by 6.5-7 weeks). Major organs form. Morning sickness typically peaks weeks 6-9.

Weeks 9-12: Fetus reaches ~3 inches by end of T1. First ultrasound usually at 10-12 weeks.

Weeks 13-16: Baby's size doubles. Many women start feeling subtle "quickening" (first felt movements) — first-time moms usually around 18-22 weeks, experienced moms earlier.

Weeks 17-20: Anatomy ultrasound (18-22 weeks). Detailed fetal anatomy, often gender determined.

Weeks 21-26: Viability threshold (around 22-24 weeks for survival outside the womb with NICU care). Baby's organs maturing rapidly.

Weeks 27-32: Third trimester begins. Glucose tolerance test (24-28). Tdap vaccine (27-36). Increasing physical discomfort.

Weeks 33-36: Group B strep test (35-37). Pre-labor symptoms can begin (Braxton-Hicks contractions, increased pelvic pressure).

Weeks 37-40: Full term. Baby can arrive any time. Hospital bag should be packed by 36 weeks.

Weeks 40-42: Post-due-date. Most providers will induce by 41-42 weeks if labor hasn't started.

A worked example

Aisha, 30, has an LMP of January 8, 2026. She's tracking her pregnancy weekly. Today is May 12, 2026 — that's 17 weeks 4 days pregnant.

This week (week 17): baby is about 5 inches long, weighing 5 ounces. Skeleton is hardening from cartilage to bone. Baby can hear sounds from outside. Her uterus has grown to about the level of her belly button area; she has a clear baby bump now and is in maternity-clothes territory. Her energy is high (T2 honeymoon trimester), morning sickness is fully gone. She may feel light "fluttering" movements — first-time experience of quickening for first-time moms typically falls between weeks 16-22.

Next milestone: anatomy ultrasound between weeks 18-22 (anywhere from May 19 to June 16 for her). At that appointment, the OB will measure every part of the baby in detail, check organ development, and likely determine gender if she wants to know. This is the most-anticipated ultrasound of pregnancy for most parents.

Three months later — Aisha is 30 weeks pregnant (August 4). Trimester 3. Baby weighs about 3 lbs, length about 16 inches. She's gained 19 lbs total. Heartburn is becoming common (uterus pushes on stomach). Movements are strong and visible from outside. Recent glucose tolerance test came back normal — no gestational diabetes. Upcoming: Tdap vaccine at her 30-week appointment, hospital tour at 32 weeks, group B strep test at 36 weeks.

Five weeks before due date: Aisha is 35 weeks pregnant (September 8). Baby is now ~5 lbs, almost full size. She has trouble sleeping comfortably; uses a pregnancy pillow. Has packed her hospital bag (per the 36-week recommendation). Increased pelvic pressure as baby "drops" — descent into the pelvis often happens 2-4 weeks before delivery for first-time moms. Her OB is now seeing her weekly. They've discussed the birth plan, anesthesia preferences, and signs of labor to watch for.

Two weeks past due date: she's 42 weeks. Baby hasn't come yet. Her OB schedules induction for Monday morning. After induction begins, she goes into active labor within 14 hours. Baby is born healthy at 7 lbs 9 oz — within normal birth weight for 42-week babies. Total pregnancy time: 294 days from LMP, 14 days past the EDD. Above the 50th percentile for first-time pregnancies but well within normal range.

Related resources

For determining your due date if it's not yet established, the Due Date Calculator. For pregnancy weight gain tracking, the Pregnancy Weight Calculator. For nutrition during pregnancy, the Calorie Calculator (with appropriate pregnancy add-ons) and Water Intake Calculator. The American College of Obstetricians and Gynecologists (ACOG) publishes patient-facing guidance for each pregnancy week and the authoritative clinical guidelines.

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Frequently asked questions

How are pregnancy weeks counted?

Medical convention counts pregnancy from the first day of your last menstrual period (LMP), not from conception. That puts the 'first week' of pregnancy at a point when you weren't actually pregnant yet. Conception happens around week 2; positive home tests show up around week 4. This is why a '40-week' pregnancy is really about 38 weeks of fetal development — the first two weeks are pre-conception by definition.

Gestational age vs fetal age — what's the difference?

Gestational age (the standard medical count) is measured from LMP and is what your OB and ultrasound technician will use. Fetal age is from conception and runs about 2 weeks behind. So a 'gestational age 12 weeks' baby has a 'fetal age' of ~10 weeks. Always use gestational age when talking to medical providers; fetal age is more of a textbook abstraction.

When does each trimester start and end?

First trimester: weeks 1–13. Second trimester: weeks 14–26. Third trimester: weeks 27 to delivery (40+ weeks). The trimester boundaries are reference points more than physiological transitions — there's no sudden change at week 14 or 27. The third trimester is often defined as 'week 28 onward' by some sources; either is fine.

How accurate is the due date?

An LMP-based due date is accurate within about ±1 week. A first-trimester ultrasound (8–13 weeks) is the most accurate — typically within ±5 days. After about 20 weeks, ultrasound dating becomes less precise because babies grow at different rates. If your dating ultrasound disagrees with your LMP by more than 7 days in the first trimester, providers usually update the due date to the ultrasound number.

What if I don't know my LMP?

Two common alternatives: (1) Use your provider's due date — they'll set one based on a dating ultrasound at your first prenatal appointment. (2) If you know your conception date (e.g., from ovulation tracking or IVF), use that instead — the calculator handles it. Either is fine; LMP is just the standard input.

Why is pregnancy measured from LMP instead of conception?

Historical and practical convenience. LMP is verifiable; conception date often isn't. Most women remember (or can look up) the start of their last period; few know the exact date of conception. The 40-week LMP convention dates to 19th-century obstetricians who needed a standardized way to predict due dates without modern testing. The 'real' pregnancy duration (conception to birth) is about 38 weeks; the LMP-based count adds the ~2 weeks from period-start to ovulation. Modern OBs sometimes use 'fetal age' or 'gestational age from conception' as alternatives, but LMP-based weeks remain the standard.

What's the difference between weeks and months in pregnancy?

Confusing, because pregnancy 'months' don't match calendar months. Common convention: pregnancy is 9 calendar months = 40 weeks = 280 days = 10 lunar months. So if someone says 'I'm 7 months pregnant,' they typically mean 28-32 weeks. The conversion is approximate: weeks 1-4 = month 1, 5-8 = month 2, 9-13 = month 3, 14-17 = month 4, 18-22 = month 5, 23-27 = month 6, 28-31 = month 7, 32-35 = month 8, 36-40 = month 9. The fuzziness is why medical providers use weeks (more precise) and patients sometimes use months (more intuitive for non-medical conversation).

When will I feel the baby move?

First-time moms typically feel movements ('quickening') between 18-22 weeks. Experienced moms often feel them earlier, around 14-16 weeks, because they recognize the sensation. Early movements feel like butterflies, bubbles, or muscle twitches — easily confused with gas at first. By 24-26 weeks, movements are unmistakable and felt from outside the body (your partner can feel them with a hand on your belly). By 28+ weeks, movements are strong and reliable. Tracking 'kick counts' starts around 28 weeks — counting movements in a defined period and noting any concerning decreases.

What weeks is morning sickness worst?

Typically weeks 6-9, with peak around week 8-9 for most women. About 70-80% of pregnant women experience some morning sickness; 35-50% experience vomiting. Most cases resolve by week 14 (end of first trimester) as the placenta takes over hormone production. Some women experience hyperemesis gravidarum — severe persistent vomiting causing weight loss and dehydration — that lasts through second trimester or longer; this requires medical management. The name 'morning sickness' is misleading; nausea can occur any time of day, and afternoon/evening nausea is just as common as morning.

What week is considered 'safe' to announce pregnancy?

Most parents wait until the end of the first trimester (week 12-13) before announcing widely. The reason: miscarriage risk drops dramatically after week 12 — from about 10-15% before week 12 to under 2% after. The 12-week mark is also typically when the first detailed ultrasound has been done, confirming viability and chromosomal screening (NIPT, if chosen). 'Announcement' decisions are personal — some parents tell close family at 6-8 weeks, friends at 10-12 weeks, and announce publicly at 14-16 weeks. There's no medically correct timeline; the social convention is roughly 12-13 weeks for public announcement.

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