Pregnancy Due Date: How It Is Calculated
· 5 min read
How doctors estimate your due date from your last period, why pregnancy is counted as 40 weeks, how cycle length changes it, and what the trimesters mean.
Naegele's rule
The standard estimate adds 280 days (40 weeks) to the first day of your last menstrual period (LMP). For example, an LMP of 1 March gives a due date of 6 December.
A quick hand method — add seven days, subtract three months — lands within a day or two of the same date.
Why 40 weeks and not 38?
Pregnancy is counted from the LMP, not from conception. Ovulation and conception usually happen about two weeks later, so the baby's actual age is about two weeks less than the 'weeks pregnant' figure.
If your cycle isn't 28 days
Naegele's rule assumes ovulation on day 14 of a 28-day cycle. With a 32-day cycle you probably ovulated around day 18, so the due date moves four days later. The PakCalc due date calculator adjusts for your cycle length automatically.
Trimesters
- First trimester: weeks 1 to 13
- Second trimester: weeks 14 to 27
- Third trimester: week 28 until birth (full term is 37–42 weeks)
How accurate is it?
Only about 1 in 20 babies is born on the due date itself; most arrive within two weeks either side. An ultrasound scan in the first trimester is the most accurate way to date a pregnancy, and your doctor may adjust the date after it.
This guide is general information, not medical advice. Please follow the advice of your doctor, LHV or midwife.