How is the due date calculated?

By Naegele's rule: add 280 days (40 weeks) to the first day of your last menstrual period. This assumes a 28-day cycle with ovulation on day 14.

Is the due date accurate?

Only about 5% of babies arrive on their exact due date. Most babies are born within 2 weeks before or after. Your provider may adjust the date after an ultrasound.

What is the difference between LMP and conception date?

LMP is the first day of your last period. Conception usually happens about 14 days later. Using conception date shifts the due date by about 14 days, so only use it if you are sure of the conception date.

A full-term pregnancy is 37 to 42 weeks. Before 37 weeks is preterm, after 42 weeks is post-term.

Due date calculator

Estimate your baby's arrival by last menstrual period (LMP) or conception date.

First day of your last menstrual period

Common questions

By Naegele's rule: add 280 days (40 weeks) to the first day of your last menstrual period. This assumes a 28-day cycle with ovulation on day 14.
Only about 5% of babies arrive on their exact due date. Most babies are born within 2 weeks before or after. Your provider may adjust the date after an ultrasound.
LMP is the first day of your last period. Conception usually happens about 14 days later. Using conception date shifts the due date by about 14 days, so only use it if you are sure of the conception date.
A full-term pregnancy is 37 to 42 weeks. Before 37 weeks is preterm, after 42 weeks is post-term.