Naegele's rule
The standard estimate adds 280 days — forty weeks — to the first day of the last menstrual period. This assumes a 28-day cycle with ovulation on day 14, so a longer or shorter cycle shifts the date accordingly.
Pregnancy is dated from the last period rather than conception, which is why you are counted as roughly two weeks pregnant at the moment of conception.
A date, not a deadline
Only about 4% of babies arrive on their estimated due date. Around 80% arrive within the two weeks either side of it, which is why clinicians treat 37–42 weeks as full term.
An early ultrasound dates a pregnancy more accurately than the last period, particularly with irregular cycles. If your clinic has given you a scan-based date, use theirs.
The trimesters
The first trimester runs to week 13 and covers organ formation, when the pregnancy is most vulnerable. The second, weeks 14 to 27, is when most people feel best and when movement usually becomes noticeable. The third runs from week 28 to birth and is dominated by growth and positioning.
Viability outside the womb begins around 24 weeks with intensive care, improving steadily thereafter. Full term is 39 to 40 weeks; 37 to 38 is early term, and past 42 is post-term.
Dating scans versus the last period
An ultrasound in the first trimester measures the embryo directly and dates the pregnancy to within a few days. Dating from the last menstrual period assumes a textbook cycle and can be off by a week or more with irregular cycles.
Where the two disagree by more than about a week, clinicians generally adopt the scan date. If your clinic has given you a date, treat it as authoritative over anything calculated here.
Frequently asked questions
Is this a substitute for medical advice? +
No. It is an arithmetic estimate. Antenatal care, dating scans and any concern about your pregnancy belong with a qualified clinician.
How accurate is the due date? +
Only about 4% of babies arrive on it. Roughly 80% arrive within two weeks either side, which is why the full-term window spans several weeks.
Can the due date change? +
Yes. Early scans commonly revise it, and a first-trimester scan is considerably more accurate than a later one for this purpose.