How the due date is calculated
A pregnancy is counted as 280 days, or 40 weeks, from the first day of the last menstrual period. This is Naegele's rule, the standard method doctors use. It assumes a 28-day cycle with ovulation on day 14, so the calculator moves the date later or earlier if your cycle is longer or shorter. For example, a last period that started on 1 January gives a due date of 8 October. Counting from the period means you are already about two weeks 'pregnant' on the day you conceive.
Dating from conception or IVF
If you know the date of conception, the due date is 266 days later. IVF dates are the most precise, because the age of the embryo is known. Following the American College of Obstetricians and Gynecologists (ACOG), the due date is 263 days after a day-3 embryo transfer and 261 days after a day-5 blastocyst transfer. If your fertility clinic gave you a due date, use theirs.
Why your doctor's date may be different
An ultrasound in the first trimester measures the baby and is the most accurate way to date a pregnancy. ACOG advises changing the due date when the early scan differs from the period-based date by more than a set number of days. Once your doctor or midwife confirms a due date, use that one for all your appointments. Only a small share of babies arrive on the exact due date. Most are born within two weeks either side of it.
Trimesters and term
The first trimester runs to the end of week 13, the second from week 14 to the end of week 27, and the third from week 28 until birth. A baby born from 37 weeks is early term, from 39 weeks is full term, at 41 weeks is late term and from 42 weeks is post-term. The key dates list shows when each stage starts for you, plus the usual windows for the mid-pregnancy anomaly scan and glucose screening for gestational diabetes. Your own clinic in the UAE, Saudi Arabia or elsewhere may book these checks on a slightly different schedule.
Your privacy
Pregnancy is private health information. This calculator runs entirely in your browser. The dates you enter are not stored, sent to a server or recorded in analytics.