👶 Due Date Calculator
How Due Dates are Calculated
Most due dates are calculated using Naegele's Rule: add 280 days (40 weeks) to the first day of your last menstrual period (LMP). This assumes a 28-day cycle and ovulation on day 14. Adjustments are made for different cycle lengths.
Pregnancy Trimesters
| Trimester | Weeks | Approx. Dates |
|---|---|---|
| First | 1–12 | Weeks 1–12 |
| Second | 13–26 | Weeks 13–26 |
| Third | 27–40+ | Weeks 27–40+ |
⚠️ Due dates are estimates — only about 5% of babies are born on their exact due date. Always confirm with your OB-GYN or midwife.
Frequently Asked Questions
The most common method is Naegele's Rule: add 280 days (40 weeks) to the first day of your last menstrual period (LMP). Alternatively, add 9 months and 7 days to the LMP date. This assumes a 28-day cycle with ovulation on day 14.
Only about 5% of babies are born on their exact due date. Most births occur within 2 weeks before or after the due date. A due date is best understood as the midpoint of a normal delivery window — roughly 38–42 weeks gestation.
If your cycle is longer than 28 days, your due date will be later; if shorter, earlier. For example, with a 35-day cycle, ovulation occurs around day 21, adding 7 days to the standard calculation. Your OB will typically use an early ultrasound to confirm the due date.
Gestational age is counted from the first day of your last period — this is the standard medical measurement. Fetal age (or conception age) is counted from fertilization, which is typically 2 weeks less than gestational age. Doctors almost always use gestational age.
First visit: 8–10 weeks. Then monthly until 28 weeks, every 2 weeks from 28–36 weeks, then weekly until delivery. Total: around 10–15 prenatal visits for a normal pregnancy. High-risk pregnancies have more frequent monitoring.