Due Date Calculator: Pregnancy Week and Trimester
By Hesaplayıcı
The due date calculator finds your estimated due date from the first day of your last period, the conception date or the day of an IVF embryo transfer. From your last period, enter your cycle length too. It also shows how many weeks pregnant you are on a day you choose, the trimester and the estimated conception date.
Worked example: Last period on 1 January 2026, a 28-day cycle: the week of pregnancy on 1 June 2026
Estimated due date
October 8, 2026
- Weeks pregnant
- 21 weeks
- Days after the full weeks
- 4 days
- Trimester
- 2
- Estimated conception date
- January 15, 2026
- Days to the due date
- 129 days
- Days pregnant
- 151 days
Step by step
days added = 280 + (cycle length − 28)
days added = 280 + (28 − 28) = 280 days
Days to the due date
days to the due date = due date − day of the calculation
days to the due date = 2026-10-08 − 2026-06-01 = 129 days
Days pregnant
days pregnant = 280 − days to the due date
days pregnant = 280 − 129 = 151 days
Weeks pregnant
weeks of pregnancy = ⌊days pregnant / 7⌋
weeks of pregnancy = ⌊151 / 7⌋ = 21 weeks
Days after the full weeks
extra days = days pregnant − 7 × weeks of pregnancy
extra days = 151 − 7 × 21 = 4 days
Trimester
trimester: under 14 weeks 1, under 28 weeks 2, later 3
trimester: 21 weeks = 2
How to use
- Under Calculate from, choose the date you have: your last period, the conception date or an IVF transfer.
- For your last period, fill in First day of the last period and Cycle length. Leave 28 if you do not know your cycle.
- For IVF, fill in Embryo transfer date and Embryo age.
- In Week of pregnancy on, enter a day; leave it empty to use today. Press Calculate.
Formula
- From the last period:
due date = first day of the last period + 280 + (cycle length − 28) days - From conception:
due date = conception date + 266 days - From an IVF transfer:
due date = transfer day + 266 − embryo age days days pregnant = 280 − days to the due date,weeks pregnant = ⌊days pregnant / 7⌋
280 days are 40 weeks. Conception is about 14 days after the first day of the last period, so 266 days are left from conception to the due date. The embryo age is 3 or 5 days: 261 days are added for a day-5 embryo and 263 for a day-3 embryo. Trimesters follow ACOG: the 1st runs to 13 weeks 6 days, the 2nd starts at week 14 and the 3rd at week 28.
Worked example
The example on the page is someone whose last period started on 1 January 2026, with a 28-day cycle. The result shows the estimated due date, the week of pregnancy on 1 June 2026 and the trimester. The steps show the days added, the days to the due date and the change to weeks.
Limits
This is an estimate, not medical advice. A first-trimester ultrasound dates a pregnancy more accurately; the date your doctor gives comes before this one. With irregular cycles, a date from the last period can be wrong. The calculator does not look at twins, the type of birth or the risk of an early birth.
Frequently Asked Questions
How is the due date calculated?
Add 280 days (40 weeks) to the first day of your last period. If your cycle is longer than 28 days, add the difference; if it is shorter, take it away. 266 days after conception gives the same date.
How is an IVF due date calculated?
Add 261 days to the transfer day of a day-5 embryo (blastocyst) and 263 days for a day-3 embryo. The rule is from ACOG Committee Opinion 700.
Why are pregnancy weeks counted from the last period?
The first day of the last period is a date most people know. Conception is about 2 weeks later, so the 40 weeks include these 2 weeks at the start.
My ultrasound gives another date. Which one is right?
Use the ultrasound date. A first-trimester ultrasound is the most accurate estimate; if your doctor gave you a date, that date wins.
Will my baby be born on the due date?
Most likely not. Most births come between weeks 37 and 42, and few babies are born on the due date itself.
Calculation rules
- The due date is 280 days (40 weeks) after the first day of the last period (ACOG Committee Opinion 700). For a cycle longer or shorter than 28 days, the difference is added or taken away, as the NHS due date calculator does.
- From a conception date, the due date is 266 days later. For IVF, 261 days are added to the transfer day of a day-5 embryo and 263 days for a day-3 embryo (ACOG Committee Opinion 700).
- The week of pregnancy counts from the first day of the last period; conception is about 2 weeks after that day. From a conception or transfer date, the calculator finds that start day backwards.
- Trimesters follow ACOG: the 1st trimester runs to 13 weeks 6 days, the 2nd starts at 14 weeks 0 days and the 3rd at 28 weeks 0 days.
- This is an estimate. Most births come between weeks 37 and 42; few babies are born on the due date. A first-trimester ultrasound is more accurate: if your doctor gave you a date from an ultrasound, use that date.
- When the day is empty, today is used: the calendar day in Türkiye time (Europe/Istanbul). If the day is before the count starts (a few days after the period with a long cycle), the due date is given without the week and the trimester.