Pregnancy Due Date Calculator
Estimate your baby's due date based on how many weeks pregnant you are. Useful at any point in pregnancy to quickly determine how many weeks remain until your estimated delivery date.
About this calculator
A full-term pregnancy lasts approximately 40 weeks from the first day of the last menstrual period (LMP). If you already know how many weeks pregnant you are, the weeks remaining until your estimated due date (EDD) is simply: Weeks remaining = 40 − weeks_pregnant. Cycle length can shift the LMP-based estimate; cycles longer than 28 days push the EDD later, while shorter cycles push it earlier, typically by (cycle_length − 28) days. Healthcare providers use Naegele's Rule as the standard method: add 280 days (40 weeks) to the LMP date. Ultrasound dating in the first trimester is considered the most accurate method, and may override LMP-based calculations if there is a discrepancy of more than 5–7 days.
How to use
Suppose you are 12 weeks pregnant and your cycle length is 30 days. Step 1: Weeks remaining = 40 − 12 = 28 weeks left until your estimated due date. Step 2: Cycle adjustment = 30 − 28 = +2 days, so your EDD shifts 2 days later than the standard LMP estimate. If your LMP was January 1, the base Naegele EDD would be October 8, adjusted to October 10. At 12 weeks, you are in your first trimester and approximately 28 weeks from delivery.
Frequently asked questions
How accurate is a due date calculator based on last menstrual period?
LMP-based due date estimates are accurate to within about 2 weeks for most women with regular cycles. Studies show that only about 5% of babies are born on their exact due date, with most deliveries occurring in a window of 38–42 weeks. First-trimester ultrasound is more precise, typically accurate to within 5–7 days. Your healthcare provider may adjust your EDD after your dating ultrasound if there is a significant discrepancy.
Why does cycle length affect my estimated due date?
Standard due date calculations assume a 28-day cycle with ovulation on day 14. If your cycle is longer, say 35 days, you likely ovulate around day 21, meaning conception occurs later and your EDD should be pushed back by approximately 7 days. Conversely, a 21-day cycle shifts the EDD about 7 days earlier. Accounting for cycle length improves the accuracy of LMP-based estimates, particularly for women with irregular or notably long or short cycles.
What is the difference between gestational age and fetal age during pregnancy?
Gestational age counts from the first day of your last menstrual period and is the standard used by doctors and midwives — at conception, gestational age is already about 2 weeks. Fetal age (also called embryonic age) counts from the actual date of fertilization and is roughly 2 weeks less than gestational age. When your provider says you are 12 weeks pregnant, they mean 12 weeks gestational age. Due date calculators and prenatal milestones all use gestational age as the reference.