Pregnancy Due Date Calculator

About this calculator

A free pregnancy due date calculator that estimates when a baby might arrive based on the date of the last menstrual period, a known conception date, or an IVF embryo transfer date. The LMP method applies Naegele's rule — adding 280 days (40 weeks) to the first day of the last period — which has been the standard clinical convention since the nineteenth century. The conception method adds 266 days to the date intercourse or insemination is believed to have occurred, and the IVF method counts forward from a day-5 blastocyst transfer (261 days) or a day-3 transfer (263 days). The calculator also shows gestational age in weeks and days from the LMP, the current trimester, and how many days remain to the estimated due date. Everything runs in your browser on the dates you enter — nothing is uploaded or stored. The result is a planning estimate, not medical advice, a clinical diagnosis, or a substitute for a dating ultrasound or prenatal care. Due dates are inherently uncertain: only about 4% of births occur on the exact predicted date, full-term delivery spans 37 to 42 weeks of gestation, and a first- or second-trimester ultrasound may adjust the estimate if the fetal measurements differ from the LMP calculation. Talk to your obstetrician, midwife, or other prenatal care provider about your individual situation.

Naegele's rule and the 280-day convention

The LMP method is the oldest and most commonly used approach. German obstetrician Franz Karl Naegele codified the rule in the early 1800s: take the first day of the last menstrual period, add one year, subtract three months, and add seven days — arithmetic that is equivalent to adding 280 days, or exactly 40 weeks. The convention assumes a 28-day menstrual cycle with ovulation on day 14, placing conception about two weeks after the LMP. That is why gestational age, counted from the LMP, is always roughly two weeks longer than fetal age counted from conception: the first two weeks of "pregnancy" by this clock occurred before fertilisation.

The 280-day rule works well as a population average, but individual cycles vary. Someone with a consistent 35-day cycle typically ovulates on day 21, not day 14, so conception occurs a week later than the formula assumes — shifting the true due date about a week later than Naegele predicts. Conversely, a 21-day cycle shifts it earlier. The calculator uses the standard 280-day formula; if your cycle length differs significantly from 28 days, mention it to your provider, as they may adjust the estimate accordingly or rely on ultrasound dating instead.

Dating ultrasounds and why they can override the LMP estimate

A first-trimester ultrasound — typically performed between 8 and 13 weeks of gestation — measures crown-rump length (CRL), the distance from the top of the fetal head to the bottom of the torso. Because fetal size is relatively uniform at this stage, CRL correlates tightly with gestational age. If the ultrasound-derived age differs from the LMP estimate by more than a few days, most clinicians will revise the estimated due date to match the ultrasound measurement, since the scan is more accurate than recalled menstrual dates.

A second-trimester anatomy ultrasound (typically at 18–20 weeks) is less reliable for dating because fetal growth becomes more variable after the first trimester. At that point, providers generally keep the due date that was established earlier. A dating change late in pregnancy is uncommon and usually requires a clear discrepancy between growth measurements and the existing estimate. This calculator cannot replicate ultrasound dating; the dates it produces should be treated as an initial estimate pending your first prenatal appointment.

Gestational age, fetal age, and the three trimesters

Gestational age is counted in weeks and days from the first day of the last menstrual period. At 12 weeks and 6 days gestational age, the fetus has actually existed for roughly 11 weeks — the gap reflects the convention of starting the clock at the LMP rather than at conception. Clinicians use gestational age universally because the LMP date is known with more certainty than the conception date. The calculator shows gestational age in weeks and days alongside the trimester.

The three trimesters divide the 40-week pregnancy into broad phases. The first trimester covers weeks 1 through 13 (0 to 13 weeks, 6 days), the second spans weeks 14 through 27 (14 weeks to 27 weeks, 6 days), and the third begins at week 28. Full-term delivery is defined as 39 weeks 0 days through 40 weeks 6 days; early term is 37 to 38 weeks, late term is 41 weeks, and post-term is 42 weeks or beyond. A due date marks the midpoint of the expected delivery window, not the endpoint — roughly half of babies arrive before and half after.

IVF and conception-date methods

When the conception date is known — either from careful tracking of ovulation or from artificial insemination — the due date can be estimated by adding 266 days (38 weeks). This is mathematically consistent with Naegele's rule: the formula assumes conception on day 14 of a 28-day cycle, so LMP + 280 = conception + 266. For a conception date of January 15, 2026, the estimated due date is October 8, 2026.

IVF transfers use a slightly different count because the embryo's age at transfer is known precisely. For a day-5 blastocyst transfer — the most common type — the calculator adds 261 days; for a day-3 transfer it adds 263 days. A day-5 transfer on February 1, 2026, gives an estimated due date of October 20, 2026. In all IVF cases, the fertility clinic typically provides its own dating and the resulting gestational age will be used by the obstetric team going forward. As with LMP-based dating, a first-trimester ultrasound may confirm or adjust the estimate.

By variant

• Due Date from Last Period
• Due Date from Conception Date

Questions

How accurate is a due date calculated from the last menstrual period?

It is a reliable estimate for planning purposes, not a precise prediction. Studies show only about 4% of births occur on the exact predicted date. Naegele's rule assumes a regular 28-day cycle with ovulation on day 14 — if your cycle is longer or shorter, the true due date may shift by several days to a week. A first-trimester dating ultrasound is generally more accurate than the LMP method and may revise the estimate. Your care provider will use all available information to set a working due date.

What is the difference between gestational age and fetal age?

Gestational age counts from the first day of the last menstrual period, which is typically about two weeks before conception. Fetal age (sometimes called embryonic or conceptional age) counts from the moment of fertilisation. At 12 weeks gestational age, the fetus has existed for roughly 10 weeks. Clinicians use gestational age universally because the LMP is usually a known or reliably recalled date, while the exact conception date is often uncertain.

What does "full term" mean?

Full term is defined as 39 weeks 0 days through 40 weeks 6 days of gestation. Early term covers 37 to 38 weeks, late term is 41 weeks, and post-term is 42 weeks or beyond. A birth before 37 completed weeks is preterm. The due date at 40 weeks sits at the midpoint of the full-term window, not at its end — roughly half of all spontaneous deliveries occur before the estimated due date and half after.

Is this calculator a substitute for prenatal care?

No. This tool produces a planning estimate only. It is not medical advice, a clinical diagnosis, or a replacement for prenatal care. Due dates can shift based on ultrasound findings, medical history, and individual factors that no online calculator can assess. Please work with a qualified obstetrician, midwife, or other healthcare provider to establish and manage your estimated due date.