How the due date is calculated
Pregnancy is counted from the first day of your last menstrual period (LMP), about two weeks before conception. A typical pregnancy lasts 280 days, or 40 weeks, from that date. Naegele's rule gives the same result by hand: add one year, subtract three months and add seven days.
Due date = LMP + 280 days + (cycle length − 28)
From conception: due date = conception date + 266 days
Worked example. Your last period started on January 1, 2026. Adding 280 days gives October 8, 2026. Naegele's shortcut agrees: January 1, 2027 minus three months is October 1, 2026, plus seven days is October 8. With a 32-day cycle you probably ovulated about four days later than average, so the estimate moves to October 12.
The cycle adjustment assumes the extra days fall before ovulation, which is usually true because the phase after ovulation is fairly constant at about two weeks.
What "term" means (ACOG)
| Term | Weeks of pregnancy |
|---|---|
| Preterm | before 37w 0d |
| Early term | 37w 0d to 38w 6d |
| Full term | 39w 0d to 40w 6d |
| Late term | 41w 0d to 41w 6d |
| Post-term | 42w 0d and later |
The trimesters are usually counted as weeks 0 to 13w 6d, 14w 0d to 27w 6d, and 28w 0d to birth.
Ultrasound and changed due dates
The American College of Obstetricians and Gynecologists says an ultrasound measurement of the embryo or fetus in the first trimester, up to 13 weeks 6 days, is the most accurate way to set or confirm the gestational age. If the ultrasound date differs from the LMP date by more than a set margin (more than 5 days before 9 weeks, more than 7 days from 9 to 13 weeks 6 days), the ultrasound date is used. Once set, the due date should rarely change. Pregnancies from IVF are dated from the embryo transfer date instead.
How often babies arrive on the due date
The due date is the middle of a wide range. In a study that tracked the exact day of ovulation in 125 natural pregnancies, Jukic and colleagues (2013) found that the length of pregnancy still varied by 37 days between women, even after excluding preterm births and complications. Plan for the full-term window, not a single day.
When to contact your care team
Book a first prenatal visit as soon as you know you are pregnant. Call your doctor or midwife right away for vaginal bleeding, severe belly pain, a severe headache or vision changes, fluid leaking, fewer baby movements later in pregnancy, or regular contractions before 37 weeks.