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Pregnancy Calculator: How Many Weeks Pregnant Am I?
Enter the first day of your last period and we'll work out how many weeks pregnant you are, which month and trimester you're in, and your estimated due date. If your cycle isn't 28 days, adjust the length for a more accurate result.
Pregnancy is counted from the first day of your last period, not from conception.
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Type your last period date or pick it from the calendar.
How Is a Pregnancy Week Calculated?
Counterintuitive as it sounds, pregnancy is counted from the first day of your last period rather than from conception. The reason is practical: it is usually impossible to know exactly which day conception happened, but the date of your last period is known. Counted this way, there is no baby yet during the first two weeks of pregnancy — that period is the run-up to ovulation.
The maths is simple: divide the number of days from the first day of your last period until today by 7. The whole number is your pregnancy week and the remainder is the day within that week. If 87 days have passed, for example, 87 ÷ 7 = 12 weeks and 3 days.
How Is the Estimated Due Date Found?
The common method is Naegele's rule: add 7 days to the first day of your last period, go back 3 months, then add 1 year. This assumes a 280-day (40-week) pregnancy and a 28-day cycle.
If your cycle is longer than 28 days, ovulation happens later and the due date shifts back by the same amount; a shorter cycle brings it forward. This tool makes that correction automatically based on the cycle length you enter.
Only a small share of births happen exactly on the estimated date. Birth between weeks 37 and 42 is medically considered normal; a due date is the middle of a range rather than a fixed day.
Why Does an Ultrasound Change the Date?
Not everyone's cycle is 28 days, and the timing of ovulation varies from person to person. That is why a calculation based on your last period isn't always exact.
An ultrasound in the first trimester measures the baby's crown-rump length (CRL), and that measurement gives a far more accurate gestational age. Your doctor may update your dates based on it — the first-trimester measurement is the most reliable one. If you have a date revised by ultrasound, use that.
Frequently Asked Questions
How do I calculate how many weeks pregnant I am?
Divide the number of days from the first day of your last period until today by 7. The quotient is your pregnancy week and the remainder is the day within that week. Pregnancy is counted from the first day of your last period, not from conception.
How is a due date calculated?
By Naegele's rule: add 7 days to the first day of your last period, go back 3 months and add 1 year. This assumes a 40-week pregnancy and a 28-day cycle; if your cycle differs, the date shifts by the same amount.
How accurate is a pregnancy calculator?
It is quite accurate for people with a regular 28-day cycle. With irregular cycles there can be a discrepancy, which is why a first-trimester ultrasound is used to firm up the date. If your doctor has revised your dates by ultrasound, go with those.
How many months is a given number of pregnancy weeks?
The commonly used mapping is: weeks 1-4 month 1, weeks 5-8 month 2, weeks 9-13 month 3, weeks 14-17 month 4, weeks 18-22 month 5, weeks 23-27 month 6, weeks 28-31 month 7, weeks 32-35 month 8, and weeks 36-40 month 9.
How many weeks does pregnancy last?
A full-term pregnancy is taken as 40 weeks. Births before week 37 are preterm and those after week 42 are post-term. Weeks 37 to 42 is the normal range.
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This content is for informational purposes only and does not replace medical advice. Always consult your doctor or midwife about your own or your baby's health.