How to use it
- Choose what you know: the first day of your last period, the conception date, or an IVF transfer date.
- Pick the date. For the last period, enter your usual cycle length (28 days if you are not sure).
- Read the due date, how many weeks pregnant you are today, and the trimester dates.
“Today” is your device’s date, so the weeks update every day.
How it works
- Last period (Naegele’s rule): due date = first day of the last period + 280 days. If your cycle is longer or shorter than 28 days, the difference is added or subtracted, a common adjustment because ovulation comes later in longer cycles.
- Conception: due date = conception + 266 days (38 weeks).
- IVF: due date = transfer + 263 days for a day-3 embryo, or + 261 days for a day-5 embryo (ACOG Committee Opinion No. 700, 2017).
From the due date the calculator goes back 280 days to the dating start and counts the weeks from it. The second trimester begins at 14 weeks and the third at 28 weeks.
Worked example
Last period started on 1 January 2026, cycle 28 days: 1 January + 280 days = 8 October 2026. On 1 March 2026 that is 59 days, or 8 weeks 3 days – the first trimester. The second trimester starts on 9 April and the third on 16 July. A 35-day cycle moves the due date 7 days later, to 15 October.
Good to know
- Irregular cycles, recent hormonal contraception or not remembering the date all make the estimate less certain. An ultrasound in the first trimester is the usual way to confirm it.
- This calculator is for information. Your doctor or midwife sets the official due date and will guide your care – contact them with any questions about your pregnancy.