Pregnancy Week Calculator

About Pregnancy Weeks

Use the pregnancy week calculator to find out when each week of your pregnancy will start, when the week ends, and how long until or since then. Pregnancy weeks are based on the start date of your Last Menstrual Period (LMP) using Naegle's Rule, the de facto standard used to estimate due date.

You can select from week 1, which is 7 days after LMP, up to week 42. Naegle's Rule is based on estimating due date on 280 days (40 weeks) from LMP. However, since most babies are are up to 2 weeks early or late, you can calculate up to pregnancy week 42.

Dating pregnancy at LMP is a factor in why estimated due date is only correct about 5 percent of the time; a recent study shows it's even less accurate at about 4 percent. But LMP does not have sufficient detail about each women's unique cycle: cycle length, regularity, luteal phase, and time until implantation. Using ovulation date to date pregnancy potentially could yield a more reliable EDD, where pregnancies might still be early or late, but off by less, but here in the 2020s LMP is still the predominant dating method used in early pregnancy.

Visit WhenMyBaby's Pregnancy Weeks Calendar for more information, and to view a completely pregnancy map of every pregnancy week.