Clinical gestational age and dating methods
In modern obstetrics, a full-term pregnancy is defined as 280 days (40 weeks + 0 days) from the first day of the last menstrual period (LMP), according to Naegele’s rule.
Because menstrual cycles vary in length, ovulation and fertilization typically occur 14 days before the next expected period rather than always on day 14. Adjusting for cycle duration provides a significantly more accurate delivery estimate.
Early ultrasound measurement of fetal crown-rump length (CRL) between 11 and 14 weeks serves as the international gold standard for pregnancy dating, correcting discrepancies of more than 5 to 7 days from LMP estimates.
Worked example: 30-day menstrual cycle
An expectant mother reports an LMP of September 1st with a typical 30-day menstrual cycle.
Standard Naegele’s rule calculates: September 1st + 280 days = June 8th.
Adjusting for the 30-day cycle (+2 days past 28-day baseline) yields an estimated due date of June 10th, with conception estimated around September 17th.
Due date inputs
Calculation Method
Select LMP, conception date, ultrasound gestational measurement, or IVF transfer.
Cycle Length
Duration in days from one period start to the next (default 28 days).
CALCULATOR QUESTIONS
Pregnancy due date & gestational weeks calculator questions
What percentage of babies are born on their exact due date?
Only about 4% to 5% of babies arrive on their exact estimated due date. Approximately 90% arrive within two weeks before or after (between 37 and 42 weeks).
When does the first trimester end?
The first trimester concludes at 13 weeks + 6 days (14 completed weeks), when major fetal organs are formed and the risk of miscarriage declines substantially.