Due Date Calculator
Find your pregnancy due date, trimesters and key milestones
How is Your Due Date Calculated?
The most common method is Naegele's rule, which adds 280 days (40 weeks) to the first day of your last menstrual period. This assumes a standard 28-day cycle with ovulation on day 14. If your cycle is longer or shorter than 28 days, this calculator automatically adjusts the due date accordingly.
If you know your conception date, your due date is calculated by adding 266 days (38 weeks) from that point. For IVF pregnancies with a 5-day blastocyst transfer, 261 days are added to the transfer date. For a 3-day transfer, 263 days are added.
How Accurate is Your Due Date?
Only around 5 percent of babies are born on their exact due date. Most births occur within two weeks either side of the calculated date. Your due date is best thought of as the midpoint of a normal delivery window rather than a precise target. Ultrasound dating in the first trimester is the most accurate method and may revise the calculated due date if your cycle is irregular.
The Three Trimesters
The first trimester covers weeks 1 to 12 and is when all major organs and structures form. The second trimester runs from week 13 to week 26 — often called the most comfortable stage as morning sickness typically eases and energy returns. The third trimester from week 27 to week 40 is when the baby gains most of its weight and prepares for birth.
Due Date Calculator — Frequently Asked Questions
How is my due date calculated?
Due date is calculated using Naegele's Rule: add 280 days (40 weeks) to the first day of your last menstrual period. This is the standard method used by obstetricians and midwives worldwide, based on an average 28-day cycle.
How accurate is an estimated due date?
Only about 5% of babies are born on their exact due date. Most births occur within two weeks either side of the estimated date. An early ultrasound before 14 weeks provides the most accurate due date by measuring the baby directly.
When does each trimester start and end?
The first trimester runs from week 1 to week 13. The second trimester covers weeks 14 to 27. The third trimester begins at week 28 and continues until birth, with full term defined as 39–40 weeks.
What if I do not know my last period date?
If you do not know your last menstrual period date, you can enter your estimated conception date instead. Your midwife or doctor can also determine gestational age more precisely using an early ultrasound measurement.