What's the difference between LMP and conception dating?

Pregnancy is conventionally measured from the first day of your last menstrual period (LMP), which is roughly two weeks before conception in a regular cycle. So at four weeks pregnant by LMP, conception happened about two weeks ago. We support all three dating methods so you can use whichever is most reliable for you: LMP, known conception date, or IVF embryo-transfer date.

How does cycle length affect the due date?

Naegele's rule assumes a 28-day cycle and ovulation on day 14. If your cycle is longer or shorter, the LMP-based estimate shifts by the same number of days. Our calculator adjusts for cycle length when you use the LMP method.

How is IVF dating different?

With IVF you know the exact day of fertilization or embryo transfer. For a day-5 (blastocyst) transfer, we add 261 days to the transfer date. For a day-3 transfer, the math is slightly different — your fertility clinic will give you the precise number. Always go with the date your clinic provides.

Why is my ultrasound due date different from my LMP date?

First-trimester ultrasound measures the embryo directly and is more accurate than LMP-based dating, especially if you have irregular cycles or are unsure of your last period. ACOG guidance is to revise the due date based on ultrasound when it differs from LMP by more than 5 to 7 days in early pregnancy.

When does the third trimester start?

The first trimester runs from weeks 1 to 12 (LMP-based), the second from weeks 13 to 27, and the third from week 28 to delivery. Babies born at 37 weeks or later are considered full-term; before 37 weeks is preterm.

Is this calculator a substitute for prenatal care?

No. A due date calculator is a planning aid. Once you have a positive pregnancy test, schedule a visit with an obstetrician, midwife, or your GP. Prenatal care, ultrasound dating, and screening tests are how pregnancies are actually managed.

Pregnancy Due Date — LMP, conception, IVF

Pregnancy Due Date Calculator

Naegele's rule, ACOG dating · Trimester and weeks pregnant

Implements a published clinical formula — see citation

Maintained by TheHealthTools team. Not medical advice.

First day of last period

Average cycle length (days)

How due-date estimates work

Pregnancy is conventionally dated from the first day of your last menstrual period (LMP). The classic rule of thumb — Naegele's rule — is to add 280 days (40 weeks) to that date. It assumes a regular 28-day cycle and ovulation on day 14. We adjust for cycle-length variation when you provide it, and we also support conception-date and IVF-transfer dating, which skip the LMP-to-ovulation assumption entirely.

For most pregnancies, an LMP-based estimate is within ±5 days of the date confirmed by first-trimester ultrasound. ACOG's current guidance is to use the ultrasound date when it differs from the LMP estimate by more than 5–7 days in the first trimester. Once your obstetric provider confirms a due date, stick with that — adjustments later in pregnancy are unusual.

Trimester guide

First trimester: weeks 1–12. Embryo organ development; first prenatal visit and dating ultrasound.
Second trimester: weeks 13–27. Anatomy scan (around 18–22 weeks). Most pregnant people feel best in this window.
Third trimester: weeks 28–40+. Fetal growth and positioning; weekly visits in the final month.

Source: American College of Obstetricians and Gynecologists. Methods for Estimating the Due Date. Committee Opinion No. 700. Obstet Gynecol. 2017;129(5):e150-e154.

Frequently asked questions

An LMP-based estimate is accurate to within ±5 days for women with regular 28-day cycles. The most accurate dating combines this with first-trimester ultrasound (especially crown-rump length between 8 and 13 weeks). Only about 4 to 5 percent of babies are born exactly on their due date — most arrive within two weeks before or after.