Pregnancy due date and gestational age calculator
Expected date of delivery (Naegele’s rule) and today’s gestational age from the LMP.
What this calculator is for
It gives the expected date of delivery (EDD) and today's gestational age from the first day of the last menstrual period (LMP). You need these at booking, to time screening and scans, and to interpret any problem in pregnancy, from bleeding to raised blood pressure.
The rule
Naegele's rule: EDD = LMP + 280 days (9 months and 7 days) for a 28-day cycle. For longer or shorter cycles, add or subtract the difference from 28 days, because ovulation happens about 14 days before the next period.
Worked example
LMP 1 January 2026 with a regular 28-day cycle: EDD 8 October 2026. With a regular 32-day cycle, the EDD moves 4 days later, to 12 October.
Interpreting gestational age
- Preterm: before 37 weeks.
- Term: 37+0 to 41+6 weeks.
- Post-term: 42 weeks or more.
Gestational age is written in completed weeks and days, for example 32+4.
When the LMP date is unreliable
- Irregular cycles, recent hormonal contraception, breastfeeding or bleeding in early pregnancy all make LMP dating inaccurate.
- A first-trimester scan measuring crown-rump length (between about 10 and 14 weeks) is the most accurate way to date a pregnancy. When scan and LMP disagree, use the scan.
- After about 14 weeks, dating uses head circumference and becomes less precise. Many women in Pakistan book late, so record which method you used.
- In IVF pregnancies, date from the embryo transfer date.
- Once the EDD is set by an early scan, do not change it with later scans; a small baby on a later scan may be growth-restricted, not "wrongly dated".
See vaginal bleeding in pregnancy, ectopic pregnancy, pre-eclampsia, gynae and obs emergency and the textbook chapter on pregnancy physiology.
Frequently asked questions
How is the due date calculated?
Add 280 days (9 months and 7 days) to the first day of the last menstrual period, and adjust for cycles longer or shorter than 28 days.
Which is more accurate, LMP or the dating scan?
A first-trimester dating scan measuring crown-rump length is more accurate. If it differs from the LMP date, use the scan date.
What counts as a term pregnancy?
From 37+0 to 41+6 weeks. Before 37 weeks is preterm and 42 weeks or more is post-term.
Should the due date change after a later scan?
No. Once the pregnancy has been dated by an early scan, keep that date. A later scan that measures small may mean growth restriction.
A calculator supports, but does not replace, clinical judgement. Check results before acting on them.