How to use it
- Choose how you know the start under Start from: the last period, conception, a known due date or an IVF transfer.
- Enter the date that matches your choice. For the last period, also enter the Cycle length.
- Read the Estimated due date, the Current week of pregnancy and the Trimester.
- Scroll the Week by week table to see the date of each week and its milestone.
How the due date is worked out
A due date is counted as 40 weeks, or 280 days, from the first day of the last period. The calendar then moves it by the cycle length minus 28 days. The method follows the rules in ACOG's Committee Opinion 700, Methods for Estimating the Due Date (2017). From conception the due date is 266 days later, which is 38 weeks. From an IVF transfer the age of the embryo is added: 263 days for a 3-day embryo and 261 days for a 5-day embryo.
The dates are estimates and not medical advice. Talk to your midwife or doctor about your pregnancy: they will confirm the due date, usually with an early ultrasound.
Last period: first day + 280 days + (cycle length - 28)
Conception: conception date + 266 days
IVF: transfer date + 263 days (3-day embryo) or + 261 days (5-day embryo)
Weeks: counted back from the due date, so the due date is 40+0Trimesters and terms
This calendar splits the pregnancy into three trimesters: the first to 13 weeks 6 days, the second from 14 weeks 0 days to 27 weeks 6 days, and the third from 28 weeks 0 days. The NHS week-by-week guide uses other boundaries, with the second trimester from week 13, so check with your midwife which dates they use.
ACOG's Committee Opinion 579, Definition of Term Pregnancy, names the stages of a birth at term: early term from 37 weeks 0 days, full term from 39 weeks 0 days, late term from 41 weeks 0 days, and post term from 42 weeks 0 days.
- First trimester: up to 13 weeks 6 days.
- Second trimester: 14 weeks 0 days to 27 weeks 6 days.
- Third trimester: from 28 weeks 0 days.
What the calendar does not replace
- An early ultrasound, which can change the due date after the start you entered.
- Your antenatal appointments: the care team schedules checks and scans, not this calendar.
- Advice on symptoms. Contact your midwife or doctor about anything that worries you.
Worked example: last period on 1 August 2026, 28-day cycle
The due date is 1 August 2026 + 280 days = May 8, 2027.
On November 10, 2026 the pregnancy is 101 days old: 14 weeks 3 days, in the second trimester, with 179 days left until the due date.
| On this date | Week | Trimester |
|---|---|---|
| November 10, 2026 | 14 weeks 3 days | Second trimester |
| May 8, 2027 (due date) | 40 weeks 0 days | Third trimester |
Frequently asked questions
How exact is a due date from my last period?
It is an estimate. An early ultrasound often gives a more reliable date, and your midwife or doctor may change the due date after the scan.
Why does my cycle length change the due date?
The rule starts from ovulation, which comes later in longer cycles. Each day your cycle is longer than 28 days moves the due date one day later.
What is the difference between a trimester and a term?
Trimesters split the pregnancy into three parts of about 13 weeks. Terms (early, full, late and post term) describe how far along a birth is, from 37 weeks.
Why does the table go up to week 42?
ACOG defines post-term pregnancy as 42 weeks 0 days and beyond. The table runs to that week so you can see the dates.
Does the calendar store my dates?
No. The dates are calculated in your browser and nothing you enter is sent to us.
Sources
- ACOG Committee Opinion 700, Methods for Estimating the Due Date (2017, PubMed)
- ACOG Committee Opinion 579, Definition of Term Pregnancy (copy on the SMFM site)
- NHS: week-by-week guide to pregnancy
Figures and text last checked on Tuesday, November 10, 2026.