Calculate Your Due Date
Based on the first day of your last menstrual period (LMP) using Naegele's rule.
Most cycles are 21–35 days. Default is 28.
Enter your last menstrual period date to calculate your due date.
How Due Dates Are Calculated
The standard method for estimating a pregnancy due date is Naegele's rule, established in the 19th century and still the baseline used in clinical practice today. The formula adds 280 days (40 weeks) to the first day of the last menstrual period.
280 days corresponds to 9 months and 7 days, or roughly 40 weeks. The reasoning is that conception typically occurs at ovulation around day 14 of a 28-day cycle, so the first two weeks of "pregnancy" by clinical convention occur before fertilization has actually happened. This makes gestational age 2 weeks ahead of fetal age.
If your cycle is regularly longer or shorter than 28 days, the calculator adjusts the estimate accordingly — ovulation (and therefore conception) shifts by the same number of days that your cycle deviates from 28.
Early ultrasound measurements, particularly the crown-rump length (CRL) scan done in the first trimester, are the most accurate way to confirm or adjust the estimated due date. Many providers will revise the due date based on ultrasound if the LMP-derived estimate differs by more than a few days.
Trimesters Explained
First Trimester
Weeks 1–13. All major organs and body structures begin to form. The risk of miscarriage is highest in this period, which is why many people wait until the second trimester before announcing widely. Key appointments: confirmation of pregnancy, blood work, dating ultrasound, and first trimester screening around weeks 10–13.
Second Trimester
Weeks 14–27. Often called the most comfortable trimester. Morning sickness typically eases, energy returns, and the bump becomes visible. The anatomy ultrasound around week 18–20 checks fetal development in detail. Movement is usually felt for the first time around weeks 16–22.
Third Trimester
Weeks 28–40+. Rapid weight gain for both baby and parent. Prenatal visits increase to weekly around week 36. "Full term" is defined as 39 weeks — babies born between 37 and 38 weeks 6 days are considered early term. Only about 5% of babies are born on the exact estimated due date.
Frequently Asked Questions
How accurate is the estimated due date?
The EDD is just that — an estimate. Only about 4–5% of babies are born on their exact due date. Most births occur within two weeks before or after the EDD. Spontaneous labor before 37 weeks is considered preterm; labor that hasn't started by 42 weeks is usually medically managed. The Days Until Calculator can help you count down to your due date if you want to track the number of days remaining.
Can I calculate the due date from conception instead of LMP?
Yes — if you know your conception date with confidence (for example, through IVF or fertility tracking), add 266 days (38 weeks) to it. That accounts for the fact that conception happens roughly 14 days into a standard cycle, which is already included when calculating from LMP.
What if I don't remember my last period?
An early ultrasound — ideally before 14 weeks — can establish gestational age more reliably via crown-rump length measurement. Your provider will use that measurement to set or confirm your due date, which may differ from the LMP-based estimate.
What is a good resource for week-by-week pregnancy information?
The most widely read pregnancy guide in the United States is What to Expect When You're Expecting by Heidi Murkoff, which organizes information by trimester and month and has been updated regularly since its first publication in 1984. Many expectant parents find it useful alongside their care provider's guidance.
