Pregnancy calculator - week by week
Calculate the current gestational week, trimester and estimated due date from your last period.
Estimate based on 40 weeks (280 days).
Approximate calculation. The exact amount is confirmed by the Social Insurance Board or the Tax and Customs Board - check the official source.
Last updated: 2026-06-03
How is the gestational week calculated?
Pregnancy is counted from the first day of the last menstrual period (LMP). A full-term pregnancy lasts about 40 weeks, or 280 days. The first trimester runs to week 13, the second from week 14 to 27, and the third from week 28. Doctors record gestational age as weeks+days: 25+5 means 25 full weeks and 5 days. A calendar-based due date is an estimate: a first-trimester ultrasound gives the most accurate dating and may shift the date. The calculator does not replace medical advice.
How it is calculated
Formula
gestational weeks are counted from the first day of the last menstrual period (LMP). Estimated due date = LMP + 280 days (40 weeks).
Example
LMP 1 January → estimated due date ≈ 8 October.
Frequently asked questions
How is the estimated due date calculated?+
The estimated due date (EDD) is found by adding 280 days (40 weeks) to the first day of your last period. This follows Naegele's rule: last period date + 1 year - 3 months + 7 days.
How accurate is the calculation?+
It is an estimate. Only about 4% of babies are born on the exact calculated date; most arrive within a couple of weeks before or after. An early-pregnancy ultrasound gives the most accurate due date.
How does gestational age differ from conception age?+
Gestational age is counted from the last period, not from conception. Conception usually happens about 2 weeks later, so the fetus is roughly 2 weeks younger than the gestational age suggests.
When do the trimesters start?+
The first trimester runs through week 13, the second from week 14 to 27, and the third begins at week 28 and lasts until birth.
Can I use the calculator if I know the exact conception or ovulation date?+
The calculator works from the last period. If you know the exact conception or ovulation date, subtract 14 days from it and enter the result in the last-period field.
How many weeks does pregnancy last?+
A full-term pregnancy lasts about 40 weeks (280 days) from the first day of the last menstrual period. Birth between weeks 37 and 42 is considered full term.
How do I count pregnancy weeks and days?+
Count the days from the first day of your last period and divide by 7: 7 days = 1 week of pregnancy. For example, 180 days = 25 full weeks and 5 days, written by doctors as 25+5. The calculator does this automatically.
How likely is birth exactly on the due date?+
Only about 4-5% of babies are born exactly on the estimated date. Most births happen within two weeks before or after it, so treat the date as a guide.