Pregnancy Due Date Calculator
Enter the first day of your last menstrual period to estimate your due date, current week of pregnancy, and trimester, based on the standard obstetric estimate (Naegele's rule).
How the Pregnancy Due Date formula works
Naegele's rule estimates a 280-day (40-week) pregnancy from the last menstrual period (LMP):
Due date = LMP + 280 days
Current gestational age in weeks is simply the number of days elapsed since the LMP, divided by 7.
Step-by-step calculation
- Take the first day of the last menstrual period as day zero.
- Add 280 days (40 weeks) to estimate the due date.
- To find the current week of pregnancy, divide the number of days since the LMP by 7.
Worked example
Last menstrual period: April 1. Due date estimate = April 1 + 280 days ≈ January 5 of the following year. If today is 10 weeks after that LMP date, the pregnancy is at 10 weeks, in the first trimester.
Frequently asked questions
How accurate is a due date calculated this way?
Naegele's rule is a standard estimate assuming a regular 28-day cycle; only about 5% of babies arrive exactly on the calculated due date. An ultrasound dating scan, when available, is generally more precise.
What if my cycle isn't 28 days?
This calculator assumes a standard 28-day cycle. If your cycles are consistently longer or shorter, your actual due date may shift earlier or later than this estimate — your healthcare provider can adjust for your specific cycle length.
What defines each trimester?
The first trimester runs through week 13, the second trimester covers weeks 14–27, and the third trimester runs from week 28 to delivery, around week 40.