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Pregnancy Due Date Calculator: How Gestational Age Is Counted and Why Your Due Date Is an Estimate

•10 min read•By KBC Grandcentral Research Team

Only about 5% of babies are born on their exact due date — yet the due date is the organizing frame for all prenatal care, maternity leave planning, and delivery decisions. Understanding how it's calculated explains why it's an estimate, why your first ultrasound may shift it by a week, and why 'full term' was quietly redefined in 2013 from 37 weeks to 39 weeks.

Pregnancy Timeline: Gestational Weeks1st Trimester (Wks 1–13)2nd Trimester (Wks 14–26)3rd Trimester (Wks 27–40)Week 0: LMP(counting starts)Wk 8: HeartbeatWk 14: NT screenWk 20: Anatomy scanWk 28: GD screenWk 37: Early termWk 40: EDDACOG Term Definitions (updated 2013)Early termFull termLate termPost term37–38+6 wks39–40+6 wks41–41+6 wks42+ weeksThe Due Date Is the Middle of a 5-Week Window

Key Takeaways

  • Gestational age starts at LMP (last menstrual period) — two weeks before conception, which is counterintuitive but universal in obstetrics
  • Naegele's Rule: add 280 days (40 weeks) to LMP — assumes 28-day cycle with ovulation at day 14
  • Early ultrasound (8–12 weeks) is more accurate than LMP for dating — crown-rump length measurement has ±5–7 day accuracy
  • Only 5% of births occur on the EDD — normal term births range from 37 to 42 weeks (a 35-day window)
  • "Full term" is now 39–41 weeks per ACOG (2013) — elective deliveries before 39 weeks carry increased NICU admission risk

Naegele's Rule: How Due Dates Are Calculated

Franz Karl Naegele proposed the rule in 1812: estimated due date = first day of last menstrual period + 280 days (or +9 months +7 days). The formula assumes a regular 28-day menstrual cycle with ovulation occurring on day 14. Since most obstetricians lack conception date, LMP is the most consistently available anchor point.

The counterintuitive result: a "week 1" pregnancy technically began during your period, before conception occurred. By the time a positive pregnancy test shows at approximately 4 weeks, the embryo is actually 2 weeks post-conception. This gap is why IVF pregnancies use a different calculation — since transfer date (equivalent to day 17 of a natural cycle) is known precisely.

Due Date Calculation Methods Compared

Naegele's Rule (from LMP)

LMP + 280 days. Accuracy: ±2 weeks. Best when cycle is regular 28 days

First Trimester Ultrasound (8–12 weeks)

Crown-rump length measurement. Accuracy: ±5–7 days. Most accurate dating method available

Second Trimester Ultrasound (18–20 weeks)

Multiple biometric measurements. Accuracy: ±10–14 days. Less accurate than first trimester

IVF Transfer Date

Transfer date + 266 days (Day 3 transfer) or +263 days (Day 5 blastocyst). Most precise

What Happens in Each Trimester

TrimesterWeeksKey DevelopmentsTypical Appointments
First1–13Organ formation, heartbeat (~wk 6), miscarriage risk highestConfirmation, blood work, first prenatal, NT screen (~wk 12)
Second14–26Anatomy complete, movement felt (~wk 16–20), sex visibleAnatomy scan (~wk 20), glucose screening (~wk 24–28)
Third27–40Lung maturation, weight gain, head engagementBiweekly then weekly visits, GBS screen (~wk 36), NST if indicated

Calculate Your Due Date

Pregnancy & Due Date Calculator

Calculate your estimated due date from LMP or conception date, see your current gestational week, and view a full trimester timeline with key milestones.

Open Pregnancy Calculator →