How your due date is calculated
A pregnancy is dated from the first day of your last menstrual period (LMP), not from the day you conceived. That sounds odd, but it is practical: most women know when their period started, while very few know the exact day an egg was fertilised. Counting from the LMP adds roughly two “extra” weeks at the start – the weeks in which your body was preparing to ovulate.
A typical pregnancy lasts 280 days (40 weeks) from the LMP. Doctors have used this figure since the 19th century through Naegele’s rule:
Naegele’s rule assumes a 28-day cycle with ovulation on day 14. If your cycles are regularly longer or shorter, ovulation shifts too, so our calculator moves your due date by the same number of days. A 32-day cycle, for example, pushes the date four days later.
Which method should you use?
| You know… | Formula used | Best for |
|---|---|---|
| First day of last period | LMP + 280 days (± cycle adjustment) | Most natural pregnancies |
| Conception / ovulation date | Conception + 266 days | Ovulation tracking, IUI |
| IVF transfer date | Transfer + 266 − embryo age | IVF and frozen embryo transfer |
| Ultrasound result | Scan date − GA on scan + 280 | Irregular cycles, uncertain LMP |
| Due date from doctor | Works backwards to today | Checking how far along you are |
If you had a scan in the first trimester and its date differs noticeably from your LMP date, doctors usually go with the scan. Measurements of the embryo between 7 and 13 weeks are remarkably consistent from baby to baby, which makes early scans the gold standard for dating.
Worked example
Priya’s last period began on 10 January 2026 and her cycles are about 30 days long.
- Standard date: 10 January 2026 + 280 days = 17 October 2026.
- Cycle adjustment: 30 − 28 = +2 days.
- Estimated due date: 19 October 2026.
On 1 June 2026 she would be 20 weeks 1 day pregnant by LMP dating – right around the time of the detailed anomaly scan.
How accurate is a due date?
A due date is an estimate at the centre of a range. Only around 4–5% of babies are born on the exact day. Most arrive within two weeks either side. Research tracking the actual day of ovulation found that even when conception timing was known precisely, natural pregnancy length varied by up to five weeks between women. First-time mothers also tend to deliver a few days later on average than women who have given birth before.
Tip: Think of your due date as a “due month”. Plan to have your hospital bag, documents and transport ready by 37 weeks.
Early, full and late term
Doctors classify birth timing using standard definitions. The calculator’s timeline marks each of these points for your dates.
| Label | Gestational age |
|---|---|
| Preterm | Before 37w 0d |
| Early term | 37w 0d – 38w 6d |
| Full term | 39w 0d – 40w 6d |
| Late term | 41w 0d – 41w 6d |
| Post-term | 42w 0d and beyond |
Want to know what is happening right now? Use the pregnancy week calculator or jump straight to the week-by-week guide.
Frequently asked questions
How is my due date calculated?
Your due date is 280 days (40 weeks) after the first day of your last period, adjusted if your cycle is regularly longer or shorter than 28 days. With IVF or a known conception date, we count 266 days from conception instead.
Can my due date change?
Yes. If an early ultrasound shows the baby measuring more than about 5–7 days different from your LMP dates, your doctor may update your due date. Once set by a first-trimester scan, it normally isn’t changed again based on later scans.
What is EDD in pregnancy?
EDD stands for Estimated Date of Delivery – the date your pregnancy reaches 40 weeks. You may also see EDC (Estimated Date of Confinement), an older term for the same thing.
Is the due date calculated from the day of conception?
No. By convention it is counted from the first day of your last period, which is about two weeks before conception in a 28-day cycle. That’s why at conception you are already considered ‘2 weeks pregnant’.
Do first babies come late?
On average, first babies are born a few days later than subsequent babies, but individual variation is much larger than this difference. Your doctor will monitor you if you go beyond 41 weeks.
Is this calculator free and private?
Yes. It is completely free and all calculations happen inside your browser. We never receive or store the dates you enter.
Medical disclaimer: The results and information on this page are estimates for general education only. They are not a diagnosis and do not replace advice from your gynaecologist, obstetrician or midwife. If you have pain, bleeding, reduced baby movements or feel unwell, contact your doctor or call 108 immediately. Read the full disclaimer.
Sources & further reading
- American College of Obstetricians and Gynecologists (ACOG). Committee Opinion No. 700: Methods for Estimating the Due Date. acog.org
- ACOG. Committee Opinion No. 579: Definition of Term Pregnancy.
- Mittendorf R, et al. The length of uncomplicated human gestation. Obstet Gynecol 1990;75:929–32.
- Jukic AM, et al. Length of human pregnancy and contributors to its natural variation. Hum Reprod 2013;28:2848–55.
- World Health Organization. WHO recommendations on antenatal care for a positive pregnancy experience (2016). who.int