Pregnancy Week by Week Calculator
Find out exactly how many weeks pregnant you are, then explore your baby’s development, size, your body’s changes, symptoms and to-dos for every single week from 4 to 40.
Baby this week
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Your body this week
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Common symptoms
This week’s to-do
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Every pregnancy is unique. Sizes, symptoms and milestone timing are typical averages, not a diagnosis or a promise. Always follow the guidance of your own midwife or doctor.
How the pregnancy week by week calculator works
Pregnancy is counted in gestational weeks, starting from the first day of your last menstrual period — not from the day you conceived. That’s the convention every clinic uses, and it’s why you’re considered about two weeks pregnant before conception even happens. A full-term pregnancy runs about 40 weeks, split into three trimesters.
This tracker does two things at once. Enter your last period or due date and it works out exactly how many weeks and days along you are today. Then you can slide through every week from 4 to 40 to see what’s developing in your baby, how big they are, what’s changing in your body, the symptoms that are common, and a simple to-do for that week. If you already know your week, just pick it and start exploring. Don’t know your due date yet? Work it out first with our Pregnancy Due Date Calculator.
Quick answer
To find how many weeks pregnant you are, count the days from the first day of your last period to today, then divide by seven. Most people are dated to a 40-week pregnancy. This calculator does that math for you and then shows your baby’s size, development and symptoms for that exact week.
The three trimesters at a glance
| Trimester | Weeks | What’s happening | Key checkpoints |
|---|---|---|---|
| First | Weeks 1–13 | Organs, neural tube and heartbeat form. Nausea and fatigue are common. | First prenatal visit; dating & NT screening |
| Second | Weeks 14–27 | Energy returns, the bump shows, and you feel first movements. | Anatomy scan (18–22w); glucose test (24–28w) |
| Third | Weeks 28–40+ | Baby gains weight and moves into position for birth. | GBS test (35–37w); weekly visits near term |
How to read your week
When a provider says you’re “12 weeks,” they mean 12 completed weeks of gestational age. So “12 weeks and 3 days” is written as 12w 3d. Your baby’s fetal age (counted from conception) is usually about two weeks less than the gestational age. The fruit-and-veg size comparisons you’ll see here are the friendly shorthand most pregnancy trackers use — they’re averages, and healthy babies vary quite a bit around them.
When your week might be adjusted
Your dating can shift after an early ultrasound. In the first trimester, all babies grow at nearly the same rate, so a crown-rump length measurement is very reliable. If the scan and your last-period date disagree by more than about a week, your provider will usually go with the scan. You can cross-check either way with our Ultrasound Gestational Age Calculator or the Crown-Rump Length Calculator.
Related maternal health calculators
This tracker is part of the Maternal Health Calculators hub. Whatever week you’re in, these companion tools help you plan the next step — each answers a different question, so choose the one that fits where you are right now.
Frequently asked questions
Count from the first day of your last menstrual period to today and divide by seven. This calculator does it automatically when you enter your last period or due date, and shows your exact weeks and days plus your trimester.
The first trimester runs through 13 weeks 6 days, the second from week 14 to 27 weeks 6 days, and the third from week 28 until birth. The colour band on the week rail shows exactly which trimester the week you’re viewing falls in.
They’re friendly averages based on typical fetal growth charts, meant to give you a sense of scale. Real babies vary quite a bit, and your provider tracks growth more precisely with ultrasound measurements.
First movements (quickening) are usually felt between weeks 16 and 22, often earlier in a second or later pregnancy. By the third trimester, movements become strong and regular, and it’s worth noting your baby’s usual pattern.
Gestational age is counted from your last period and is the standard clinics use. Fetal age is counted from conception and is usually about two weeks less. When your provider gives you a week, they mean gestational age.
The mid-pregnancy anatomy scan is usually done between 18 and 22 weeks. It checks your baby’s organs and growth in detail and is often when you can find out the sex if you’d like to.
Viability — a real chance of survival with intensive care — is generally considered to begin around 24 weeks, though outcomes improve steadily with each additional week. The lungs continue maturing well into the third trimester.
A pregnancy is full term from 37 weeks. Babies born between 37 and 42 weeks are considered on time. Only about 5% arrive on the exact due date, so think of week 40 as the middle of a window.
Yes. Work out your due date first with the IVF Due Date Calculator, then enter that due date here (or slide to your known week) to see the week-by-week development. IVF gives especially precise dating.
No. It’s an educational guide to help you understand what’s typical for each week. It doesn’t replace appointments, scans or advice from your midwife or doctor, who monitor your individual pregnancy.
Methodology
This tracker calculates gestational age by counting days from the first day of the last menstrual period (or backward from a known estimated due date, which is the last period plus 280 days) and dividing by seven to give completed weeks and days. Each week from 4 to 40 is mapped to typical fetal length and weight ranges, common developmental milestones, maternal changes and symptoms drawn from standard obstetric references, and the usual screening and care checkpoints. Trimester boundaries follow the common clinical split: first trimester through 13 weeks 6 days, second trimester 14 through 27 weeks 6 days, and third trimester from 28 weeks. Sizes and symptoms are population averages and vary between pregnancies; this tool is educational and is not a diagnosis or a substitute for care from a qualified provider.
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References & trusted sources
- ACOG — How Your Fetus Grows During Pregnancy
- NHS — Week-by-week guide to pregnancy
- MedlinePlus — Pregnancy
- World Health Organization — Maternal health
Last reviewed & updated: July 2026.