Only about 4–5% of babies are born on their exact due date; most arrive between 37 and 42 weeks. Your doctor or midwife will confirm the date with an ultrasound.
Method: Naegele's rule, adjusted for cycle length. Results are estimates and are not medical advice.
How Your Due Date Is Calculated
The standard method is Naegele's rule: add 280 days (40 weeks) to the first day of your last menstrual period (LMP). The rule assumes a 28-day cycle with ovulation around day 14. If your cycles are longer or shorter, ovulation shifts by the same number of days, so the calculator moves the due date and your week count by the difference. For example, a 32-day cycle pushes the due date 4 days later.
Example: if your last period started on January 1, 2026 and your cycle is 28 days, your estimated due date is October 8, 2026.
Pregnancy Weeks and Trimesters
- First trimester: weeks 1 to 13. The baby's organs form during this period.
- Second trimester: weeks 14 to 27. Most people first feel the baby move during these weeks.
- Third trimester: week 28 until birth.
Pregnancy weeks are counted from the first day of your last period, not from conception. That is why conception usually happens at the end of week 2, and why you are already considered about 4 weeks pregnant when a home test first turns positive.
Frequently Asked Questions
How do I know how many weeks pregnant I am?
Count the days from the first day of your last period to today and divide by 7. The calculator does this for you, adjusts for your cycle length and shows the result in weeks and days, such as 12 weeks 3 days.
How accurate is my due date?
It is an estimate. Only a small share of babies arrive on the exact date, and birth anywhere from 37 to 42 weeks is considered normal. An ultrasound in the first trimester gives the most reliable date, and your provider may adjust your due date based on it.
What if my periods are irregular?
With irregular cycles, a date based on your last period can be off by a week or more. In that case your provider will usually date the pregnancy from the baby’s size measured on an early ultrasound.
Why is pregnancy 40 weeks and not 9 months?
Forty weeks is counted from your last period, about two weeks before conception. That adds up to roughly 9 calendar months plus one week, which is why the two descriptions do not match exactly.