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.
Three examples: what the calculator shows for different dates
The results below are calculated as of 14 September 2026, exactly as the calculator does when you press "Today". They show how the same formula paints a very different picture at different stages of pregnancy.
- Last period started 20 July 2026: 56 days have passed, 8+0, trimester 1, progress 20%, estimated due date 26 April 2027.
- Last period started 10 March 2026: 188 days have passed, 26+6, trimester 2, progress 67%, estimated due date 15 December 2026.
- Last period started 20 December 2025: 268 days have passed, 38+2, trimester 3, progress 96%, estimated due date 26 September 2026.
How to read the result correctly
The most common confusion is between completed weeks and the ordinal week. A result of 26+6 means 26 weeks are complete and you are in your 27th week. If a pregnancy app says "week 27" and the calculator says "26 weeks 6 days", both describe the same day.
The "Weeks remaining" row counts only whole weeks: at 26+6 it shows 14, although 13 weeks and 1 day are actually left until 40+0. The percentage is the share of 280 days that has passed, so 50% is reached exactly at 20+0.
The "Date (as of)" field also lets you look ahead. With a last period on 10 March 2026, you are still 12+6 and in trimester 1 on 8 June 2026, but on 9 June 2026 you reach 13+0 and the calculator switches to trimester 2. This is a quick way to find the date a particular week falls on, for example a scheduled scan.
Naegele's calendar rule and 280 days do not always match
By hand, people often count: last period date + 1 year - 3 months + 7 days. The calculator adds exactly 280 days instead. Because months differ in length, the two methods can disagree by 1-3 days.
For a last period on 1 January 2026, both give 8 October 2026. For 10 March 2026, the calendar rule gives 17 December 2026 and 280 days gives 15 December 2026. For 1 May 2026 the gap is largest: 8 February 2027 by the calendar rule versus 5 February 2027 by 280 days, because a short February falls in between.
Neither is "more correct", since birth happens within a window of several weeks anyway. If the date in your antenatal record differs from the calculator by a couple of days, this or an ultrasound correction is usually the reason.
Long or short cycles, IVF and an unknown date
The calculation assumes ovulation happened about 14 days after the period started, as in a 28-day cycle. If your cycle is consistently 35 days, ovulation happens roughly a week later and the pregnancy is younger than the calendar suggests. In that case an early scan often moves the due date later.
With IVF the conception date is known precisely and the clinic sets the due date. To follow the weeks in this calculator, enter a last-period date 19 days before a day-5 embryo transfer, or 17 days before a day-3 transfer.
If you cannot remember the date of your last period, or your cycle was irregular after stopping hormonal contraception or while breastfeeding, the calendar calculation has little value. An ultrasound is then the only reliable basis.
Common mistakes when entering the date
Most wrong results come from the input, not the formula. Check the following before making plans based on the result.
- Entering the last day of the period. The correct date is the first day of normal bleeding; otherwise the pregnancy appears 4-6 days younger.
- Mistaking early-pregnancy spotting for a period. If your last "period" was noticeably lighter and shorter than usual, the previous date is probably the right one, and it is worth telling your midwife.
- Not updating the calculator after an ultrasound correction. If the doctor gave a new due date, subtract 280 days from it and use that as the last-period date so the weeks match your record.
- If the last-period date is later than the as-of date, the calculator shows no result and asks you to check the dates.
When not to rely on the calculator
The calculator only counts days and does not assess how the pregnancy is going. Contact your midwife, doctor or emergency services straight away if you have bleeding, severe abdominal pain, leaking fluid, sudden severe swelling with a headache or vision problems, or if the baby's movements clearly decrease in the second half of pregnancy.
After 40+0 the calculator shows 100% progress and 0 weeks remaining but keeps counting (for example 42+1). Going past the due date is common and usually no reason to panic, but your doctor decides the monitoring plan. The calendar figure is a guide; decisions belong to a healthcare professional.
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.