How Pregnancy Due Dates Are Calculated

Starting With the Last Menstrual Period

Most due date estimates begin with the first day of the last menstrual period (LMP). Even though conception has not yet occurred at that point in the cycle, dating from LMP gives a consistent reference that patients can usually recall and providers can record without any additional testing.

A standard gestational length of 280 days — or 40 weeks — is then added to the LMP date to arrive at the estimated due date (EDD). This figure comes from decades of clinical observation rather than a single biological constant. It reflects the average time between a woman's LMP and delivery across large populations, not an exact physiological limit.

Naegele's Rule Explained

The most common formula applied to LMP dates is Naegele's rule, named after the 19th-century German obstetrician Franz Karl Naegele. The rule is: add one year, subtract three months, and add seven days to the first day of the LMP.

Mathematically, this is equivalent to adding 280 days — the same result as the direct day addition. The three-step formulation was useful in an era without digital calculators because it is easy to do with a calendar: find the month three months before the LMP, add a year, then add seven days.

The assumption underlying Naegele's rule is a 28-day cycle with ovulation on day 14. For individuals with consistently longer or shorter cycles, providers may adjust the EDD. A person with a 35-day cycle would be expected to ovulate around day 21, not day 14, shifting the estimated conception date and therefore the due date forward by roughly a week.

How Ultrasound Refines the Estimate

An early first-trimester ultrasound — typically done between weeks 8 and 12 — measures the fetus to establish gestational age independently of the LMP date. The primary measurement is crown-rump length (CRL), the distance from the top of the head to the base of the spine. This measurement correlates closely with gestational age in early pregnancy.

If the ultrasound-derived age differs from the LMP-based estimate by more than a set threshold (often five to seven days in the first trimester), many clinical guidelines recommend updating the EDD to match the ultrasound finding. Ultrasound dating is generally considered more accurate than LMP dating because it does not depend on cycle regularity or accurate recall of the LMP date.

Second and third-trimester ultrasounds are less accurate for dating because individual variation in fetal size increases substantially as pregnancy progresses. A difference of two weeks in late pregnancy may simply reflect normal variation in fetal size rather than an error in dating.

The EDD as a Window, Not a Point

A due date is a midpoint estimate, not a guaranteed delivery date. Full-term pregnancy is defined as 39 to 40 weeks of gestation. The range of "normal" delivery extends from 37 weeks (early term) through 42 weeks (late term), a five-week window centered roughly on the EDD.

About 80 percent of spontaneous deliveries occur within two weeks of the estimated due date in either direction. Only a small percentage of pregnancies deliver exactly on the calculated date. Expecting an exact prediction sets up unnecessary anxiety; understanding the EDD as the center of a probability distribution is more accurate and more helpful.

Post-term pregnancy — defined as gestational age beyond 42 weeks — carries higher risks for both parent and baby. Providers typically monitor closely after 40 weeks and may discuss induction options if the pregnancy continues past 41 or 42 weeks. The precise threshold varies by clinical guideline and individual circumstances.

Other Dating Methods

Assisted reproductive technology (ART)

When pregnancy follows in vitro fertilization (IVF) or intrauterine insemination (IUI), the date of egg retrieval or embryo transfer is known precisely. The EDD for IVF pregnancies is typically calculated from the egg retrieval date plus 266 days, or from the embryo transfer date adjusted for the age of the embryo at transfer. LMP-based formulas are not used because the timing of conception is documented.

Fundal height measurement

From around 20 weeks onward, a provider can measure the distance from the pubic bone to the top of the uterus (fundal height) in centimeters. From 20 to 36 weeks, this measurement roughly equals the gestational age in weeks. Fundal height is a screening tool, not a dating method, but large discrepancies may prompt an ultrasound review.

Using Online Calculators Responsibly

Online due date calculators are convenient tools for getting an initial estimate and planning milestones like prenatal appointments, maternity leave, and nursery timelines. A reliable calculator applies Naegele's rule to the LMP date and may allow you to adjust for cycle length.

The output is always an estimate. It should be confirmed and potentially adjusted by a healthcare provider who can factor in ultrasound findings, individual health history, and clinical judgment. No calculator substitutes for professional prenatal care, and any concerns about dates, symptoms, or fetal movement should go to a qualified clinician rather than a website.