How is the due date calculated?
A pregnancy lasts about 40 weeks (280 days), counted from the first day of the last period, not from conception. That is why in the first two weeks you are not yet pregnant, even though they are counted.
- From the last period: add 280 days. If your cycle is longer or shorter than 28 days, add or take away the difference.
- From conception: add 266 days.
- From an IVF transfer: add 261 days for a day-5 embryo, or 263 days for a day-3 embryo.
The three trimesters
| Trimester | Weeks |
|---|---|
| First | Week 1 to the end of week 13 |
| Second | Week 14 to the end of week 27 |
| Third | Week 28 to birth |
Worked example
Last period started on 1 May 2026, with a 28-day cycle
1 May 2026 + 280 days = 5 February 2027. On 4 October 2026 you are 22 weeks and 2 days pregnant, in the second trimester.
Only a small share of babies arrive exactly on the due date. Most are born between weeks 37 and 42. Your doctor may change the date after an early ultrasound, which is more accurate than any calculation.
Frequently asked questions
How accurate is the due date?
It is an estimate. Most babies are born within two weeks before or after it. An ultrasound in the first trimester gives the most accurate date, so follow your doctor’s date if it differs.
What if I don’t know the date of my last period?
Use the conception date if you know it, or ask your doctor for a dating ultrasound, which can estimate how far along you are.
Why does the calculator show the Hijri date?
Many families in the Gulf plan with the Hijri calendar, so the due date is also shown in the Umm al-Qura calendar used in Saudi Arabia.
Do I need to tell my employer?
Labour laws in the Gulf give paid maternity leave: 12 weeks in Saudi Arabia and 60 days in the UAE. Tell your employer early so the leave can be planned.
Last reviewed: October 2026. This calculator gives an estimate for planning and does not replace medical advice. See a doctor for regular check-ups, and get help straight away for bleeding, severe pain or reduced baby movements.
This page was translated from Arabic. If something is unclear or wrong, please tell us.