Three ways to calculate a due date
- LMP (Naegele's rule): first day of last period + 280 days. The standard for most pregnancies. We adjust for cycle length — a 32-day cycle pushes the due date ~4 days later.
- Conception date: conception + 266 days. Useful if you tracked ovulation directly.
- IVF transfer: the most precise — Day-3 transfer + 263 days, Day-5 transfer + 261 days.
What if my due date is wrong?
The most accurate due date comes from a first-trimester ultrasound — specifically the crown-rump-length (CRL) measurement at 8–13 weeks. If the ultrasound differs from your LMP-based date by more than 7 days, clinicians use the ultrasound date. The ACOG recommends never changing an early-pregnancy ultrasound date once it's been set, even if later scans suggest different sizing.
How likely is it that you deliver on the due date?
Only 4–5% of babies arrive on their exact due date. But 80%+ arrive within 10 days either side, and ~95% arrive between 37 and 42 weeks. The “due date” is really a midpoint of a 5-week normal window, not a deadline.
Trimesters at a glance
- First trimester (weeks 1–12): organs forming, miscarriage risk highest. Folic acid, hydration, sleep, early prenatal visit.
- Second trimester (weeks 13–26): usually the most comfortable phase. Anatomy scan around week 20.
- Third trimester (weeks 27–40): rapid growth, viability reached at ~24 weeks, full term at 37. Glucose tolerance test, group B strep screen, weekly visits in the last month.
When to call your clinician
- Heavy bleeding or severe abdominal pain
- Persistent severe headache, vision changes, or upper-belly pain (signs of preeclampsia)
- Decreased fetal movement after 28 weeks
- Leaking fluid before 37 weeks
- Persistent vomiting that prevents keeping fluids down
References
- ACOG Committee Opinion 700. Methods for estimating the due date. Obstet Gynecol. 2017;129:e150-4.
- Jukic AM et al. Length of human pregnancy and contributors to its natural variation. Hum Reprod. 2013;28(10):2848-55.