🤰 Pregnancy Due Date Calculator

Last updated: April 1, 2026

🤰 Pregnancy Due Date Calculator

Estimate your due date using Naegele's rule — the same method used in clinical practice

Enter the first day of your most recent period. If your cycle length differs from 28 days, consult your provider for an adjusted estimate.

What does your due date actually mean — and how certain is it?

When a healthcare provider tells you your due date, there's an unspoken asterisk attached to that date. The figure you receive — let's say October 8 — is a statistical estimate, not a booking confirmation. Only about 5% of babies arrive on their precise due date. Most births occur in a two-week window straddling it. Knowing this upfront saves a lot of unnecessary anxiety in those final weeks when the calendar feels like a countdown clock but your body seems to have different ideas.

The due date itself is calculated using Naegele's rule, a formula developed by German obstetrician Franz Karl Naegele in the 1800s. Despite its age, it remains the starting point for virtually every OB and midwife globally. The math is simple: take the first day of your last menstrual period (LMP), add 280 days, and that's your estimated due date (EDD). Those 280 days equal 40 weeks — the standard length of a full-term pregnancy measured from the LMP, even though conception typically happens about two weeks after that date.

Why count from your period instead of when you actually conceived?

This trips up a lot of people. If you became pregnant two weeks after your period started, why does the gestational clock start from the period itself? The answer is practical, not biological.

Most people know the date their last period started. Very few know exactly when they ovulated or conceived — sperm can survive in the fallopian tubes for up to five days, and the egg is viable for 12–24 hours, so the fertilization window is fuzzy even for couples tracking carefully. Because the LMP is a concrete, knowable date, medicine standardized around it. The result is that when your provider says you're "10 weeks pregnant," the embryo is closer to 8 weeks old in terms of actual development. Gestational age and embryonic age are genuinely different numbers.

What happens at each trimester?

Trimesters are a convenient way to organize an otherwise enormous span of developmental changes. They're not sharp biological transitions — more like rough chapters.

First trimester (weeks 1–13): This is the period of foundational construction. The neural tube forms, the heart begins beating around week 6, and all major organ systems are laid down in miniature. It's also the period with the highest miscarriage risk, which is why many people choose to wait before making announcements. Nausea and fatigue are common because hCG (human chorionic gonadotropin) levels surge rapidly — that's the hormone pregnancy tests detect. Symptoms often peak around weeks 8–10 then ease off as the placenta takes over hormone production around week 12.

Second trimester (weeks 14–26): For many people, this is the most comfortable stretch. The miscarriage risk drops substantially, energy often returns, and the pregnancy becomes visibly apparent. Around week 18–20, movement is typically felt for the first time — a fluttery sensation called quickening. The anatomy scan, usually around week 20, provides a detailed look at fetal development and can identify the sex if desired. Growth accelerates; by week 24, the fetus reaches the threshold of viability outside the womb with intensive medical support.

Third trimester (weeks 27–40): The final stretch involves rapid weight gain, lung maturation, and the fetus settling into position for birth. Sleep becomes harder, pressure on the bladder increases, and Braxton Hicks contractions — practice contractions — become noticeable. Around week 36, providers check fetal position. Most babies turn head-down naturally by this point. From week 37 onward, a birth is considered "full term," though weeks 39–40 are ideal for fetal lung and brain development.

Can your due date change after it's been set?

Yes, and it happens fairly regularly. The most common reason is a first-trimester ultrasound that shows a fetus measuring differently than the LMP-based calculation predicts. If the discrepancy is more than about 5–7 days, providers typically adjust the EDD to match the ultrasound measurements, because early ultrasound dating is more accurate than LMP-based calculation alone.

Several factors can make LMP-based dating less precise. Irregular cycles are the big one — Naegele's rule assumes a 28-day cycle with ovulation on day 14. If your cycle runs 35 days and you ovulate on day 21, your actual EDD is 7 days later than a standard calculation suggests. Breastfeeding, hormonal contraceptives taken until recently, or conditions like PCOS can all affect cycle regularity and therefore dating accuracy.

IVF pregnancies are a different situation entirely. When embryo transfer date is known, dating is done from that date using a simple offset — typically adding 266 days from the date of conception (or adjusting by embryo age at transfer). There's no LMP ambiguity to contend with.

What does "40 weeks" look like in practice for most pregnancies?

Statistically, about 80% of spontaneous births happen between weeks 37 and 42. The 40-week mark is the peak probability, not a deadline. Many hospitals and midwifery practices now discuss induction around 41 weeks, with the evidence suggesting that waiting past 42 weeks (called post-term) carries increased risks related to placental function and fetal size.

For a pregnancy dated to 40 weeks today, the realistic window for labor to start naturally is roughly week 38 through 41 — a span of three weeks. Viewing the EDD as the midpoint of a range rather than a specific target date is genuinely helpful for mental preparation.

A note on the calculator you just used

The calculation here adds exactly 280 days to your entered LMP date, which faithfully implements Naegele's rule. It shows gestational age in weeks and days, which is how healthcare providers describe pregnancy — you'll hear your provider say "you're 32 weeks and 4 days" rather than "you're in your eighth month." The trimester boundaries used (weeks 1–13, 14–26, 27–40) reflect standard clinical definitions, though you'll occasionally see slightly different cutoffs in different sources.

If there's any uncertainty about your LMP — or if your cycles are irregular — treat this result as a starting estimate and confirm the due date with an early ultrasound. That combination of LMP dating refined by ultrasound is how modern obstetric care establishes the most accurate EDD possible.

FAQ

What is Naegele's rule and how accurate is it?
Naegele's rule adds 280 days (40 weeks) to the first day of your last menstrual period to estimate your due date. It's reasonably accurate when your cycles are regular and close to 28 days, but studies suggest only about 5% of babies are born on the exact due date. First-trimester ultrasound combined with LMP dating gives the most accurate result — if there's more than a 5–7 day difference between the two, providers typically adjust to the ultrasound measurement.
Why does gestational age start from the last period rather than conception?
Because the exact date of conception is rarely known with certainty — sperm can survive several days in the fallopian tubes, making fertilization timing imprecise. The first day of the last menstrual period is a concrete, memorable date that most people can report accurately, so medicine standardized gestational age around it. The result is that 'gestational age' runs about two weeks ahead of true embryonic age.
My cycles are longer than 28 days. Will this calculator give me the right date?
Standard Naegele's rule assumes a 28-day cycle with ovulation on day 14. If your cycle is consistently longer — say, 35 days — you likely ovulate around day 21, which means conception happens about a week later than the formula assumes. Your actual due date would be approximately 7 days later than this calculator shows. Some providers use an adjusted Naegele's formula: add the difference between your cycle length and 28 days to the calculated EDD. A first-trimester ultrasound will clarify this.
What is the difference between full term, early term, and post-term?
These terms reflect when during the final weeks a baby is born. Early term is weeks 37–38, full term is weeks 39–40, late term is week 41, and post-term is week 42 or beyond. Babies born at 39–40 weeks generally have the best outcomes for lung maturity, feeding, and brain development. Births before 37 weeks are considered preterm. Many providers now discuss elective induction around 41 weeks given evidence that risks begin to increase slightly past that point.
Does my due date change if I had an IVF transfer?
Yes — IVF pregnancies use the known transfer date rather than an LMP, since the timing of fertilization is documented precisely. For a day-5 blastocyst transfer, the standard calculation sets the EDD at 261 days after transfer (equivalent to a 38-week calculation from a 'virtual' LMP). For a day-3 embryo transfer, it's 263 days. Your fertility clinic typically provides the adjusted EDD based on your specific transfer protocol.
What are gestational weeks versus trimester months — why does pregnancy seem to last '10 months'?
The common '9 months' figure is a rough approximation. A full-term pregnancy is 40 weeks, and calendar months average about 4.3 weeks — making it closer to 9.3 calendar months, or 10 lunar (28-day) months. This is why some people say pregnancy is '10 months long.' Healthcare providers track pregnancy in weeks rather than months precisely because of this ambiguity — 'week 28' is a specific, unambiguous point in development, whereas 'seventh month' can mean different things depending on how you count.