Pregnancy Due Date Calculator

Key Pregnancy Milestones

Term Classification Windows (ACOG/WHO)

Calendar dates for each ACOG/WHO term category computed from your established due date. The highlighted row shows where today falls.

What is a Pregnancy Due Date?

A pregnancy due date, also known as the estimated date of delivery (EDD), is the predicted date when a pregnant woman is expected to give birth. This date is calculated to be approximately 280 days (40 weeks) from the first day of the woman's last menstrual period (LMP).

It's important to note that only about 5% of babies are born on their exact due date. Most babies are born between 37 and 42 weeks of pregnancy, which is considered full term. The due date is an estimate and serves as a reference point for tracking the pregnancy's progress and planning prenatal care.

Calculation Methods

• Last Menstrual Period (LMP): The most common method, using Naegele's rule: add 280 days (40 weeks) to the first day of your last menstrual period. This method assumes a regular 28-day cycle with ovulation occurring on day 14.
• Conception Date: If you know when conception occurred (or likely occurred), the due date is calculated by adding 266 days (38 weeks) to that date. This method is particularly useful for those who used fertility treatments or know the exact date of conception.
• Ultrasound Dating: The most accurate method, especially when performed in the first trimester (weeks 8-13). The ultrasound measures the size of the fetus to determine gestational age and calculate the due date. This method is often used to confirm or adjust the due date calculated from LMP.

Pregnancy Trimesters

• First Trimester (Weeks 1-13): This period covers conception through week 13. Major organs and structures begin to form. Morning sickness, fatigue, and breast tenderness are common symptoms.
• Second Trimester (Weeks 14-27): Often called the 'golden period' of pregnancy. Energy levels typically improve, morning sickness subsides, and you may start to feel baby's movements. The baby's gender can usually be determined during this time.
• Third Trimester (Weeks 28-40+): The final stretch before birth. The baby gains most of their weight during this period. You may experience increased discomfort, Braxton Hicks contractions, and frequent urination as the baby grows and prepares for birth.

Important Notes

• Only about 5% of babies are born on their exact due date - it's an estimate, not a guarantee
• Full-term pregnancy is considered anywhere between 37 and 42 weeks
• First-time mothers tend to deliver slightly later on average than women who have given birth before
• Ultrasound dating in the first trimester is considered the most accurate method for determining due date
• Your healthcare provider may adjust your due date based on ultrasound measurements and other factors
• Regular prenatal checkups are essential for monitoring both mother and baby's health throughout pregnancy
• Contact your healthcare provider if you experience unusual symptoms, severe pain, bleeding, or decreased fetal movement

Why does the LMP method add 280 days when pregnancy lasts about 38 weeks?

Because LMP-based dating uses the first day of your last period as Day 0, not the day of conception. Conception usually happens about 14 days after LMP (ovulation in a standard 28-day cycle), so 280 days from LMP equals about 266 days from actual conception. This is Naegele's rule, formalized by German obstetrician Franz Naegele in 1812. The original rule (LMP + 7 days − 3 months + 1 year) gives the same result. Modern OB-GYNs still use 280 days because LMP date is much easier to remember than the exact ovulation day, and on a population basis, the average error is under 5 days. If your cycle is shorter than 28 days, ovulation happens earlier and the actual gestation will be shorter by the cycle difference.

How does cycle length actually affect the due date?

Because Naegele's rule assumes a 28-day cycle with ovulation on Day 14. If your cycle is consistently 32 days, ovulation happens around Day 18 — 4 days later than the standard assumption — so your actual due date is 4 days later than the LMP+280 calculation. Conversely, a 25-day cycle shifts the due date 3 days earlier. The calculator's cycle-length input adjusts for this. Note that cycle length affects only the LMP method; the conception and ultrasound methods are unaffected because they don't rely on assumed ovulation timing. For irregular cycles (varying more than ±7 days), the LMP method is unreliable and a first-trimester ultrasound dating scan should be used instead.

Which dating method is most accurate: LMP, conception date, or ultrasound?

First-trimester ultrasound (especially weeks 8-13) is the gold standard, with accuracy of ±5 days based on crown-rump length measurement. ACOG and the International Society of Ultrasound in Obstetrics and Gynecology recommend ultrasound dating in the first trimester for all pregnancies because it removes the variability of remembered LMP and cycle length. Conception date is theoretically as accurate as ultrasound if you know the exact date (e.g., IVF or single intercourse around ovulation), but most people don't. LMP is least accurate (±7-10 days for regular cycles, more for irregular) but is the most commonly used because it's available immediately at the first prenatal visit. If a first-trimester ultrasound differs from the LMP estimate by more than 7 days, OBs switch to the ultrasound date.

Why are only 5% of babies born on their actual due date?

Because birth timing is biologically variable — the due date is the median of a distribution, not a target. Studies analyzing millions of births show that, with accurate dating, only about 4-5% of babies are born on Day 280, 50% are born within ±5 days of it, 80% within ±10 days, and 95% within the 37-42 week full-term window. The 2013 Jukic et al. study (NIH-funded, tracked 125 women from conception via daily hormone tests) found that even with perfectly known conception dates, the variation in actual pregnancy length was 37 days (5 weeks) between the shortest and longest pregnancies. Factors that shift the distribution: first pregnancy (typically 3-5 days longer), maternal age, ethnicity, BMI, and the baby's individual physiology.

What's the difference between gestational age and embryonic age?

Gestational age counts from the first day of LMP. Embryonic age (or fetal age, post-conception age) counts from actual conception, which is typically 2 weeks later. So a pregnancy that is "10 weeks gestational age" is actually "8 weeks since conception". Doctors universally use gestational age in clinical practice because LMP is the more reliable reference point, but embryologists and IVF clinics often use embryonic age. The 280-day pregnancy length and trimester boundaries (13/27/40 weeks) are all in gestational age. When you read about "the embryo's heart starts beating at 6 weeks", that's 6 weeks gestational = 4 weeks since conception.

What counts as full-term, preterm, and post-term?

Per ACOG and the WHO 2013 update: early term = 37 weeks 0 days to 38 weeks 6 days (slight risk increase compared to full term); full term = 39 weeks 0 days to 40 weeks 6 days (lowest risk); late term = 41 weeks 0 days to 41 weeks 6 days (induction often discussed); post-term = 42 weeks 0 days or beyond (rare, ~3% of pregnancies; usually induced due to rising stillbirth and placental insufficiency risk). Preterm = before 37 weeks (about 10% of births globally), with subcategories: extremely preterm (under 28 weeks, ~5% of preterm), very preterm (28-32 weeks), and moderate-to-late preterm (32-37 weeks). The shift from "36 weeks 6 days is term" to the current more granular classification happened in 2013 after evidence that even early-term babies have measurably more complications.

Can the due date change during pregnancy?

Yes, and it's clinically routine. Most often the date is revised after the first-trimester dating scan (usually weeks 8-13) if it differs from the LMP estimate by more than 5-7 days. After 14 weeks, ACOG generally recommends keeping the originally established due date even if later ultrasounds suggest a different size, because fetal growth variability increases as pregnancy progresses — a small baby at 28 weeks doesn't mean conception happened later, it usually means the baby is constitutionally small. Important note: once your OB "locks in" the EDD (estimated due date), they use that date for all subsequent decisions (timing of screenings, induction at post-term, etc.), even if it disagrees with later measurements.

Why do twin pregnancies have different due dates?

Because twins are at much higher risk for preterm birth and are routinely delivered earlier. ACOG recommendations: dichorionic-diamniotic twins (most common, fraternal twins or some identical twins with separate placentas) are typically delivered at 38 weeks; monochorionic-diamniotic twins (identical, sharing a placenta but separate sacs) at 36-37 weeks; monochorionic-monoamniotic twins (sharing both placenta and sac, rare) at 32-34 weeks due to high cord-entanglement risk. The natural full-term gestation for twins averages 36 weeks (vs 40 for singletons). Triplets average 32-33 weeks. The calculator's 40-week target applies only to singleton pregnancies — if you're pregnant with multiples, your OB will provide a different, individualized due date.

When is 39 weeks, full term, and the earliest date I can be induced?

This tool now outputs the exact calendar dates for each ACOG/WHO term window from your due date, so you do not have to count weeks by hand. Early term begins at 37 weeks 0 days (LMP + 259 days), full term at 39 weeks 0 days (LMP + 273 days), late term at 41 weeks 0 days (LMP + 287 days), and post-term at 42 weeks 0 days (LMP + 294 days). These dates drive real prenatal scheduling: elective inductions and cesareans without medical indication are generally not performed before 39 weeks 0 days (the start of full term), membrane sweeps are often offered from 39-40 weeks, induction is routinely discussed once you reach late term (41w0d), and delivery is recommended by 42 weeks 0 days because stillbirth and placental-insufficiency risk rise sharply post-term. The term-window table highlights which window today falls in.

I already know my due date — how do I find my LMP or conception date?

You can reverse the standard formulas. To get your last menstrual period (LMP), subtract 280 days (40 weeks) from your due date — for example, a due date of December 1 implies an LMP around February 25 of the same year. To get the approximate conception/ovulation date, subtract 266 days (38 weeks) from the due date, which is the same as LMP + 14 days for a standard 28-day cycle. If your cycle is not 28 days, shift the conception estimate by the cycle difference (a 32-day cycle ovulates about 4 days later). The simplest way to use this calculator for a reverse lookup is to pick the Conception method and try dates until the output due date matches the one you already have; the equivalent LMP is then shown in the milestones table.