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    How Due Dates Are Calculated: LMP, Ultrasound, IVF, and Why They Disagree

    Three different methods can give due dates a week apart. Here's how each works, when to trust which, and why your due date might shift.

    AI Calculator SuiteApril 23, 20267 min read

    The Most Famous Estimate in Medicine

    A pregnancy due date is the most quoted, least accurate number in obstetrics. About 5% of babies are born on their due date. The rest arrive in a roughly four-week window — typically 37 to 42 weeks. Yet that single date drives prenatal scheduling, maternity leave, baby-shower timing, and emotional preparation.

    Why so much fuss for an estimate? Because every week of pregnancy has expected developmental milestones, and dating helps the medical system know when to look for what. Below is exactly how that estimate is calculated.

    Method 1: Naegele's Rule (LMP-Based)

    This is the formula that powers most due-date calculators, including ours:

    > Due date = First day of last menstrual period (LMP) + 280 days

    Or equivalently: LMP + 40 weeks.

    Why 280 days from LMP?

    Conception happens roughly 14 days after LMP (around ovulation). Actual gestation from conception is about 38 weeks. Add the 2 pre-conception weeks and you get the famous 40-week count from LMP.

    This is why pregnancy is described as "40 weeks" but biology only does ~38 of them.

    When LMP works

  1. You have regular 28-day cycles
  2. You're certain of your LMP date
  3. You weren't on hormonal birth control just before

    4. When LMP fails

  4. Irregular cycles — if your cycle is 35 days, you ovulated late, and the LMP estimate runs early
  5. Recent contraceptive use — first cycle off the pill is often abnormal
  6. Stress, illness, or breastfeeding — can shift ovulation by days or weeks

    7. For these cases, ultrasound dating is more reliable.

    Try the LMP-based estimate yourself with our Pregnancy Due Date Calculator.

    Method 2: Ultrasound Dating

    In the first trimester (weeks 8–13), ultrasound measures the embryo or fetus's crown-rump length (CRL). CRL grows on a very predictable curve at this stage — accurate to within ±5–7 days.

    After 13 weeks, ultrasound dating becomes progressively less accurate because individual babies grow at different rates. By the third trimester, ultrasound estimates can be off by ±3 weeks.

    When to use ultrasound dating

  7. Irregular cycles — gold standard
  8. LMP unknown or unreliable
  9. Conception after stopping birth control
  10. First-trimester scan disagrees with LMP by more than 7 days — most providers will switch to the ultrasound date

    11. If your provider revises your due date after an early ultrasound, that's why.

    Method 3: IVF Transfer Date

    For pregnancies via in-vitro fertilization, the conception window is known exactly:

    > Due date = embryo transfer date + (266 − transfer day)

    For a Day 5 embryo (blastocyst): transfer date + 261 days.

    For a Day 3 embryo: transfer date + 263 days.

    This is the most precise dating method available. There's no LMP estimation, no early-growth variability — just a known clock.

    If you've done IVF, use the transfer date your clinic provided rather than LMP.

    Why Methods Disagree

    You might see three different due dates for the same pregnancy:

    SourceHypothetical due date
    LMP-based (cycle 32 days, ovulated late)October 15
    First-trimester ultrasoundOctober 19
    IVF transfer date(not applicable)

    A 4-day spread is normal. Bigger gaps usually mean LMP was inaccurate (irregular cycle, missed period). Most providers go with the ultrasound date when there's a meaningful disagreement.

    What "Full Term" Actually Means

    The American College of Obstetricians and Gynecologists revised its terminology in 2013:

  11. Preterm: before 37 weeks 0 days
  12. Early term: 37w 0d – 38w 6d
  13. Full term: 39w 0d – 40w 6d
  14. Late term: 41w 0d – 41w 6d
  15. Post-term: 42w 0d and beyond

    16. Most providers schedule induction at 41–42 weeks if labor hasn't started, because risks (placental aging, low amniotic fluid) increase past 42 weeks.

    Trimester Math

    TrimesterWeeksWhat's happening
    1st1–13Organ formation; highest miscarriage risk; morning sickness peaks
    2nd14–27"Honeymoon" trimester; anatomy scan ~20 weeks; first kicks
    3rd28–birthRapid weight gain; positional discomfort; final preparations

    Calculate exactly where you are with our Pregnancy Due Date Calculator — it shows trimester, current week, and milestones automatically. For tracking conception or fertile windows, see our Conception Calculator and Ovulation Calculator.

    What to Take from All This

  16. Your due date is an estimate. Don't plan around it down to the day.
  17. Trust early ultrasound over LMP when they disagree by more than a week.
  18. IVF dates are the most accurate when applicable.
  19. Anywhere from 37 to 42 weeks is normal full-term. Going "past" 40 weeks is not unusual.
  20. Use the calculator for planning, not for clinical decisions. Always defer to your prenatal provider.

    21. A due date is the start of a window, not a deadline. Babies arrive when they're ready.

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