Due Date Calculator
Enter the first day of your last period to estimate your due date and see how far along you are.
How this due date is calculated
This calculator uses Naegele's rule, the standard method: your estimated due date is 280 days (40 weeks) after the first day of your last menstrual period. If your cycle is longer or shorter than 28 days, the date shifts by the difference, because ovulation happens later or earlier in the cycle.
An early ultrasound can refine the date. If your care provider gives you a different due date, follow theirs.
What your result means
Pregnancy is counted from the first day of your last period, not from conception. That means you are already about two weeks "pregnant" when you conceive, and four weeks pregnant around the time a period would be late. The weeks and days shown in your result follow this standard counting, so they match what your doctor or midwife will use.
The milestone table shows when each stage begins:
- First trimester: from the start of pregnancy to the end of week 13.
- Second trimester: week 14 to the end of week 27. Many people have their anatomy scan around 18 to 22 weeks.
- Third trimester: week 28 until birth.
- Early term starts at 37 weeks, full term at 39 weeks and late term at 41 weeks. A pregnancy that reaches 42 weeks is called post-term.
When a scan changes your due date
A due date based on your period assumes ovulation happened at the expected time. An ultrasound in the first trimester measures the baby directly and is the most accurate way to date a pregnancy. If the scan gives a date that differs noticeably from the one based on your period, your provider will usually switch to the scan date and keep it for the rest of the pregnancy.
Frequently asked questions
How accurate is an estimated due date?
It is an estimate. Only around 4% of babies are born on their due date, and most arrive between 37 and 42 weeks.
What if I do not remember my last period?
Your doctor or midwife can date the pregnancy with an ultrasound, which is most accurate in the first trimester.
When is a baby considered full term?
Full term runs from 39 weeks to the end of 40 weeks. Babies born at 37 or 38 weeks are early term, 41 weeks is late term, and 42 weeks or later is post-term. A baby born before 37 weeks is preterm.
Can my due date change?
It can change once, usually after an early ultrasound if the scan date differs from the date based on your period. After that, providers normally keep the same due date even if later scans measure bigger or smaller.
How is the due date worked out after IVF?
With IVF the due date comes from the embryo transfer date and the age of the embryo. For a 5-day embryo it is 261 days after transfer, and for a 3-day embryo it is 263 days after transfer. Your fertility clinic will confirm it.
Sources
- ACOG Committee Opinion 700: Methods for Estimating the Due Date
- ACOG Committee Opinion 579: Definition of Term Pregnancy
- NHS: Due date calculator and pregnancy dating