How the pregnancy calculator works
Pregnancy is dated from the first day of your last menstrual period (LMP). A full-term pregnancy lasts 280 days (40 weeks) from that day — the rule doctors call Naegele’s rule.
The calculator turns whatever you know into the same starting point, then counts forward to today:
- Last period: due date = LMP + 280 days, adjusted by how much your cycle differs from 28 days (a 32-day cycle adds 4 days).
- Due date: if your provider already gave you one, we count back 280 days.
- Conception date: due date = conception + 266 days.
- IVF transfer: due date = transfer date + 266 days − the embryo’s age (3, 5 or 6 days).
- Ultrasound: due date = scan date + (280 days − the gestational age measured at the scan).
Want only the date your baby is due? Use the due date calculator. Want to know when you conceived? Try the conception calculator.
Gestational age vs fetal age
Your “weeks pregnant” is your baby’s gestational age, counted from your LMP. The baby’s actual age since conception — the fetal age or embryonic age — is about two weeks less. So at “6 weeks pregnant” the embryo is about 4 weeks old. Ultrasound reports, prenatal tests and pregnancy apps all use gestational age, which is why this calculator does too.
Pregnancy trimesters by week
| Trimester | Weeks | What’s happening |
|---|---|---|
| First | 0 – 13w6d | All major organs begin to form. Heart activity is usually seen on ultrasound by 6–7 weeks. Nausea and tiredness are common. |
| Second | 14w0d – 27w6d | Many people feel the first movements between about 16 and 22 weeks. The anatomy scan is usually done at 18–22 weeks and glucose screening at 24–28 weeks. |
| Third | 28w0d – birth | The baby gains most of their weight and the lungs mature. Group B strep testing happens at 36–37 weeks. Full term begins at 39 weeks. |
How many months pregnant am I? Weeks to months
Pregnancy is about 9 months, but because months are 4–5 weeks long, 40 weeks don’t divide evenly. This is the conversion most charts use:
| Month | Weeks pregnant | Trimester |
|---|---|---|
| Month 1 | Weeks 1–4 | First |
| Month 2 | Weeks 5–8 | First |
| Month 3 | Weeks 9–13 | First |
| Month 4 | Weeks 14–17 | Second |
| Month 5 | Weeks 18–22 | Second |
| Month 6 | Weeks 23–27 | Second |
| Month 7 | Weeks 28–31 | Third |
| Month 8 | Weeks 32–35 | Third |
| Month 9 | Weeks 36–40 | Third |
How accurate is a pregnancy calculator?
A date calculated from your last period assumes ovulation about 14 days before your next period. If your cycles are irregular, longer or shorter than average, or you’re unsure of your LMP, the estimate can be off by a week or more.
According to the American College of Obstetricians and Gynecologists (ACOG), an ultrasound measurement of the baby (crown-rump length) in the first trimester is the most accurate way to date a pregnancy. If it differs from your LMP date by more than about 5–7 days before 9 weeks, your provider will usually change your due date to match the scan. IVF pregnancies are the exception: the transfer date gives an exact date.
Remember: a due date is an estimate. Only a small share of babies — around 4% — are born on that exact day; most arrive in the weeks just before or after.
Not sure you’re pregnant yet? See when to take a pregnancy test for the earliest reliable day, or track your cycle with our ovulation calculator.
Frequently asked questions
How many weeks pregnant am I?
Count the weeks from the first day of your last menstrual period (LMP) to today. If your last period started 10 weeks and 3 days ago, you are 10 weeks and 3 days pregnant — even though conception happened about two weeks after that date. Enter your LMP above and the calculator does the count for you.
Why are pregnancy weeks counted from my last period, not conception?
Most people know when their last period started but not the exact day they ovulated or conceived. So doctors count “gestational age” from the LMP, which adds about two weeks before conception. A pregnancy is 40 weeks (280 days) from LMP, or about 38 weeks (266 days) from conception.
How accurate is a pregnancy calculator?
A date based on your last period is an estimate. It is most accurate if your cycles are regular and you know your LMP for sure. An ultrasound in the first trimester measures the baby and is the most accurate way to confirm or adjust your due date. Pregnancies conceived by IVF can be dated exactly from the transfer date.
How many months pregnant am I at 20 weeks?
At 20 weeks you are about 4½ months pregnant, which most charts call the fifth month. Because months have 4–5 weeks, there is no single official conversion; the weeks-to-months table on this page shows the usual one.
What weeks are each trimester?
Using ACOG’s definitions, the first trimester runs through 13 weeks 6 days, the second from 14 weeks 0 days to 27 weeks 6 days, and the third from 28 weeks 0 days until birth.
When is a baby considered full term?
ACOG defines early term as 37w0d–38w6d, full term as 39w0d–40w6d, late term as 41w0d–41w6d and post-term as 42 weeks or later. Babies born before 37 weeks are preterm.