How accurate is a due date?

Only about 4% of babies are born on their due date. 80% arrive within the 'term' window of 37-42 weeks; about 60% arrive within two weeks of the due date. It's best thought of as the center of a bell curve, not a deadline. First-time mothers average about 8 days past due date, subsequent pregnancies average closer to 3 days past.

Which is more accurate, LMP or ultrasound dating?

Ultrasound dating in the first trimester (especially weeks 7-13) is considered most accurate — typically within 3-5 days. LMP dating assumes a 28-day cycle with ovulation on day 14, which doesn't hold for everyone. If an early ultrasound date differs by more than 5-7 days from your LMP date, obstetricians usually revise the due date to match the ultrasound.

What if I don't know my last period?

An early dating ultrasound can establish gestational age and due date within days of accuracy. If the pregnancy is discovered later (second trimester), ultrasound is still reasonably accurate but within a wider window (±2 weeks by 20 weeks). Talk to your provider about getting an early dating scan if LMP is uncertain.

Does a later due date change anything medically?

Yes. Timing of prenatal tests (NT scan, anatomy scan, glucose tolerance test, group B strep, NIPT) is based on gestational age. If the due date is revised by a week or more, the test timing shifts accordingly. Labor inductions for post-dates are also dated from the revised due date.

Can twins or IVF change the due date formula?

Twins follow the same basic dating but are frequently delivered at 36-38 weeks (spontaneously or by induction) due to higher complication rates. IVF pregnancies have the most accurate due date of all because conception date is known exactly — count forward 266 days (38 weeks) from egg retrieval or transfer, depending on the IVF protocol.

Pregnancy Due Date Calculator: How It's Actually Calculated

Your pregnancy due date is essentially an educated guess based on a 200-year-old formula. Here's how it's actually calculated, why ultrasound dating sometimes overrides it, and what to do with the number once you have it.

Naegele's rule: the 200-year-old formula

Franz Karl Naegele, a German obstetrician, formalized the modern due-date formula in the early 1800s:

Due date = first day of your last menstrual period (LMP) + 280 days

Or equivalently: LMP + 9 months + 7 days. This assumes:

A 28-day menstrual cycle
Ovulation on day 14
Fertilization around day 14
A 38-week (266-day) gestation from fertilization

Add two weeks of pre-ovulation cycle time to 38 weeks of gestation and you get the 40-week / 280-day convention that runs from LMP, not from conception.

Get your own estimated due date with the

pregnancy due date calculator

Why the formula is approximate

Naegele's rule assumes every person has a textbook 28-day cycle, ovulating reliably on day 14. In reality:

Typical cycles range 21-35 days, with most people falling in a 25-30 day window.
Ovulation timing can shift by days based on stress, travel, illness, and weight changes.
A longer cycle (say 32 days) means later ovulation (roughly day 18) and therefore a later actual conception — pushing the "real" due date later than the LMP-based estimate.

For a 35-day cycler, Naegele's rule might put the due date a full week earlier than reality. This is why ultrasound dating matters.

Ultrasound dating

A transvaginal ultrasound in the first trimester (weeks 7-13) measures the crown-rump length (CRL) — the length of the embryo from crown to bottom — and uses established growth curves to estimate gestational age.

First-trimester ultrasound accuracy:

Weeks 7-9: accurate within ~3 days
Weeks 10-13: accurate within ~5 days
Second trimester: within ~1 week
Third trimester: within 2-3 weeks (too late to be a primary dating tool)

If ultrasound dating differs from LMP by more than 5-7 days, the ACOG (American College of Obstetricians and Gynecologists) typically recommends adjusting the due date to match the ultrasound. Early ultrasound is especially useful for people with irregular cycles or uncertain LMP dates.

How due date matters

The due date drives several medical decisions:

Test timing. First-trimester screening (weeks 11-13), anatomy scan (weeks 18-22), glucose tolerance test (weeks 24-28), group B strep screening (weeks 35-37), NIPT (week 10+).
Preterm vs term. Born before 37 weeks is preterm and may require NICU care. 37-42 weeks is term.
Post-dates management. Most OBs recommend induction by 41-42 weeks due to increasing risk of stillbirth and placental insufficiency past 42 weeks.

Only 4% of babies are born on the due date

Babies don't read calendars. Actual distribution:

~60% born within 1 week of due date (week 39-40)
~90% born within 2 weeks of due date (week 38-42)
~4% born exactly on due date
~10% born early (under 37 weeks)
~5% born past 42 weeks without intervention

First-time mothers average about 8 days past due date; subsequent pregnancies average closer to 3 days past. If your baby is "late," it's probably just normal variation, not a problem.

Worked example: LMP vs ultrasound, which wins?

Say your last menstrual period started on January 1. Using Naegele's rule, your due date would be October 8 (280 days later). At your dating ultrasound at 9 weeks, the sonographer measures a CRL that corresponds to 8 weeks and 4 days — three days less than the LMP calculation suggests.

Because the difference is only 3 days (well under the 5-7 day threshold), your provider will likely keep the LMP-based due date of October 8. The ultrasound confirms you're in the right ballpark.

Now imagine instead the CRL corresponds to 7 weeks and 5 days — 9 days behind the LMP estimate. That's past the 5-7 day threshold, so your provider would likely revise your due date to approximately October 17. Why does this matter? Your anatomy scan would shift from being scheduled at 20 weeks (per old due date) to 20 weeks per new due date — about 9 days later. The NIPT, glucose test, and group B strep timing all shift accordingly.

Use the due date calculator to find your LMP-based estimate, then bring it to your first appointment for comparison against the ultrasound.

IVF pregnancies: the most precise due date of all

In IVF (in vitro fertilization), the exact moment of egg retrieval and embryo transfer is known to the hour. This makes due date calculation more precise than any other method:

Day 3 embryo transfer: add 263 days (37 weeks and 6 days) from the transfer date.
Day 5 blastocyst transfer: add 261 days (37 weeks and 4 days) from the transfer date.

These dates reflect 38 weeks of gestation (266 days) counted from fertilization, which occurred 3 or 5 days before transfer. In IVF pregnancies, LMP is usually a fabricated date — providers back-calculate it from the transfer date for convention, but the transfer-based calculation is considered authoritative.

Practical tips for the due date

Tell people "early/mid/late [month]" instead of a specific day. Managing the flood of "any-day-now?" texts is exhausting in week 41.
Pack the hospital bag by week 36. 8-12% of babies arrive before 37 weeks; you don't want to be packing at 2 AM during labor.
Book maternity leave with a buffer. Start leave a few days before the due date to have time to rest; be prepared to extend or come back later than planned.
Know the signs of labor and when to call. Regular contractions 5 minutes apart for an hour, water breaking, significant bleeding — call your provider.

What "term" actually means

Obstetric guidelines now divide "term" into three windows because outcomes differ meaningfully within what used to be called a single category:

Early term: 37-38 weeks 6 days. Lungs and brain are still maturing. Babies born in this window have higher rates of NICU admission, feeding difficulty, and respiratory issues than those born at 39-40 weeks. Elective deliveries are generally discouraged before 39 weeks unless medically indicated.
Full term: 39-40 weeks 6 days. The target window. Outcomes are best here for both parent and baby in uncomplicated pregnancies.
Late term: 41-41 weeks 6 days. Outcomes remain good but monitoring increases. Induction is often discussed.
Post-term: 42 weeks and beyond. Placental function begins to decline. Induction is typically recommended by most providers by 42 weeks to reduce risk of stillbirth.

Understanding these windows helps make sense of your provider's recommendations as you approach and pass your due date. A discussion about induction at 41 weeks is routine and evidence-based, not a sign that something has gone wrong.

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