How to use the due date calculator
- Choose what you know: the first day of your last menstrual period (most common), the conception date, or an IVF embryo transfer date.
- Enter the date and, for the period method, your usual cycle length.
- See your estimated due date, how many weeks pregnant you are, your trimester and a timeline of key scans and dates.
How the due date is worked out
Pregnancy is counted from the first day of the last period, about two weeks before conception. A typical pregnancy lasts 280 days (40 weeks) from that day. This is known as Naegele's rule.
Due date = conception date + 266 days
Due date = 3-day embryo transfer + 263 days, or 5-day transfer + 261 days
If your cycles are longer than 28 days you probably ovulated later, so the due date moves later by the same number of days.
Example
Your last period started on 1 August 2026 and your cycle is 28 days. The estimated due date is 8 May 2027. With a 32-day cycle it moves to 12 May 2027.
How accurate is a due date?
Only about 4–5% of babies are born on their due date. Most arrive between 37 and 42 weeks. In many countries a first-trimester ultrasound (often called the dating scan, usually between 10 and 14 weeks) gives the most accurate date, and your midwife or doctor may adjust your due date after it.
Trimesters
- First trimester: weeks 1 to 13 (up to 13 weeks + 6 days).
- Second trimester: weeks 14 to 27.
- Third trimester: week 28 until birth.
Read what usually happens in each stage in our guide pregnancy week by week.
Frequently asked questions
I don't remember the date of my last period. What can I do?
An early ultrasound can estimate how far along you are. Contact your doctor or midwife to arrange your first appointment.
Why does the timeline start before conception?
By medical convention, pregnancy weeks are counted from the last period, so you are already "2 weeks pregnant" around the time of conception.
Does the Hijri due date matter?
It is shown for convenience for families who use the Islamic calendar. Medical care always uses the Gregorian date.
Is my information stored?
No. The calculation runs only in your browser.
Sources: American College of Obstetricians and Gynecologists (ACOG), "Methods for Estimating the Due Date", Committee Opinion 700; NHS, "Your pregnancy and baby guide"; WHO recommendations on antenatal care (2016). Schedules for scans and tests differ by country; follow your local maternity service.
Guides
More health calculators
Last reviewed: 2026-11-09. These tools give general information, not medical advice. Talk to a doctor about your health. See our medical disclaimer.