Pregnancy Due Date Calculator
Estimated due date from last menstrual period or conception date
Enter your last menstrual period date to instantly calculate your estimated due date, current pregnancy week, and trimester.
How Due Dates Are Calculated
Naegele's Rule
The standard method for calculating your estimated due date (EDD) adds 280 days — exactly 40 weeks — to the first day of your last menstrual period. This is called Naegele's rule and has been the clinical standard for over 150 years. It assumes a regular 28-day cycle with ovulation on day 14.
Trimesters and Pregnancy Weeks
Pregnancy is divided into three trimesters. The first trimester covers weeks 1 through 12, the second trimester spans weeks 13 through 26, and the third trimester runs from week 27 to week 40. Most full-term pregnancies deliver between weeks 39 and 41.
How Accurate Is the Due Date?
Only about 4% of babies are born on their exact estimated due date. A delivery anywhere from 2 weeks before to 2 weeks after the EDD (weeks 38–42) is considered normal. Ultrasound measurements in the first trimester can give a more precise estimate than the LMP method alone. This tool is for informational purposes only — always consult your healthcare provider for medical guidance.
EDD = LMP + 280 days (40 weeks) | Naegele's rule: LMP + 9 months + 7 days Frequently Asked Questions
How is my estimated due date calculated?
Your EDD is calculated using Naegele's rule: add 280 days (40 weeks) to the first day of your last menstrual period. If you enter a conception date instead, the tool estimates LMP as 14 days before conception, then applies the same formula.
Can I use this calculator if my cycles are irregular?
Naegele's rule assumes a 28-day cycle. If your cycles are longer or shorter, your actual due date may differ. In that case, an early ultrasound is the most reliable way to determine gestational age and due date.
What is the difference between LMP and conception date?
The LMP (last menstrual period) date is the first day of your last period before you became pregnant. Conception typically occurs about 14 days later, during ovulation. Both methods produce the same EDD because the 14-day offset is built into the formula.
How do I know which trimester I am in?
The first trimester is weeks 1–12, the second trimester is weeks 13–26, and the third trimester is weeks 27–40. This calculator shows your current trimester automatically based on today's date and your LMP.
Is it normal if my baby doesn't arrive on the due date?
Yes — only about 4% of babies are born on their exact due date. A delivery between 38 and 42 weeks is considered full term. Doctors monitor closely after 40 weeks to ensure the baby's health.
What is the formula for calculating a pregnancy due date?
EDD = LMP + 280 days (40 weeks). Using Naegele's rule: EDD = LMP + 9 calendar months + 7 days. For example, if your LMP is January 1, your EDD would be October 8 of the same year.