How is the due date calculated?
The estimated due date is calculated using Naegele's rule — 280 days (40 weeks) are added to the first day of the last menstrual period. This rule has been used since 1812 and is the most widely used method worldwide.
What is the due date formula?
Naegele's formula: Due date = Last menstrual period date + 280 days. Alternative calculation: add 7 days to the last period date, subtract 3 months and add 1 year. For example, if your last period was 2026-01-15, the estimated due date is 2026-10-22.
What are the pregnancy trimesters?
Pregnancy is divided into three trimesters:
- First trimester (weeks 1–12): The baby's major organs begin to form. This is the most critical period for development.
- Second trimester (weeks 13–26): The baby grows rapidly and begins to move. Many women feel their best during this period.
- Third trimester (weeks 27–40): The baby finishes maturing and prepares for birth. The body prepares for delivery.
How are pregnancy weeks counted?
Pregnancy weeks are counted from the first day of the last menstrual period, not from the moment of conception. Therefore, the first two weeks of pregnancy are actually before conception. A full pregnancy lasts 40 weeks (280 days), but normal delivery occurs between weeks 37 and 42.
When does delivery occur?
Only about 5% of babies are born on their exact due date. Normal delivery occurs between weeks 37 and 42 of pregnancy. First-time mothers often give birth later than the estimated date. A doctor can determine a more accurate due date based on ultrasound measurements.
What if I know my conception date or had IVF treatment?
If you know the exact date of conception (for example, from timed intercourse or fertility tracking), add 266 days to that date to estimate your due date.
For IVF pregnancies, the calculation depends on embryo age at transfer:
- Day-3 embryo transfer: Add 263 days to the transfer date
- Day-5 (blastocyst) transfer: Add 261 days to the transfer date
How accurate is the pregnancy due date calculator?
Naegele's rule assumes a regular 28-day cycle. If your cycle is longer or shorter, the estimated date shifts accordingly — roughly 1 day for each day your cycle differs from 28. First-trimester ultrasound (weeks 8–12) is the most accurate dating method because the fetus grows at a predictable rate during this window; later ultrasounds are less precise as individual growth variation increases. Always consult your doctor for personalized assessment and monitoring.