Due Date Calculator: How Doctors Actually Date Your Pregnancy

A pregnancy due date is a working clinical estimate, not a prediction of the baby's exact birthday. The familiar starting calculation adds 280 days—40 weeks—to the first day of the last menstrual period. Doctors and midwives then consider cycle history, assisted-conception dates and, especially, early ultrasound.

The result is one best obstetric estimate used to time care. Once that date is well established, it should not keep moving every time a later scan produces a slightly different measurement.

Try the free Egspect due date calculator for an immediate LMP estimate, or use the pregnancy week calculator to orient yourself on the trimester timeline. Then confirm the official date with your qualified maternity team.

Medical note: This guide provides general information, not medical advice. Pregnancy dating affects screening, growth assessment and birth planning. Use the estimated due date recorded by the clinical team responsible for your care.

TL;DR: which pregnancy date should you use?

Question	Direct answer
What is the standard due-date formula?	Add 280 days, or 40 weeks, to the first day of the last menstrual period.
What if the menstrual date is uncertain?	Early ultrasound can establish or confirm gestational age.
Is a first-trimester scan useful for dating?	Yes. ACOG describes it as the most accurate ultrasound period for establishing gestational age.
What about IVF?	Use embryo age and transfer date, normally provided by the fertility clinic.
Can a later growth scan change the date?	Usually not without a specific clinical reason; later measurements are less precise for redating.
Will birth happen on the due date?	Probably not exactly. The date anchors care within a normal birth window.

Why does pregnancy dating start before conception?

Gestational age is traditionally counted from the first day of the last menstrual period, or LMP. In a typical 28-day cycle, conception occurs around two weeks later, so the calendar's first two “pregnancy weeks” precede fertilization.

This convention is practical because the first day of menstrual bleeding is often easier to identify than the exact moment of ovulation or fertilization. It gives clinicians, laboratories and families a common way to schedule tests and describe development.

The model assumes:

• a reliable LMP date;
• a cycle close to 28 days;
• ovulation around day 14; and
• menstrual bleeding that was a true period.

Real cycles vary. Ovulation often occurs about 14 days before the next period, not automatically on cycle day 14. Irregular periods, recent hormonal contraception, breastfeeding, bleeding in early pregnancy or uncertain recall can all weaken an LMP estimate.

Read ovulation and the fertile window explained if the “day 14” shortcut is making the calendar more confusing.

How does the 280-day LMP calculation work?

The calculation is sometimes called Naegele's rule:

1. Identify the first day of the last menstrual period.
2. Add 280 days, or 40 weeks.
3. Consider whether the usual cycle is consistently shorter or longer than 28 days.

For example, an LMP on August 1 gives an estimate approximately 40 weeks later. A consistently 31-day cycle may shift a rough menstrual estimate about three days later because ovulation may occur later than the 28-day model assumes.

An online calculator performs the arithmetic; it cannot verify whether the inputs represent the biology of that cycle. Use it to prepare questions and understand timing, not to overwrite a clinical record.

The NHS due-date guidance describes pregnancy as normally lasting from 37 to 42 weeks from the first day of the last period. That range is the more useful mental model: a due date sits within a window rather than acting as an expiration date.

Why is early ultrasound important for pregnancy dating?

Ultrasound estimates gestational age from embryo or fetal measurements. Dating precision is strongest early in pregnancy because early growth follows a relatively consistent pattern; biological size differences widen later.

ACOG's Methods for Estimating the Due Date identifies first-trimester ultrasound through 13 weeks and 6 days as the most accurate way to establish or confirm gestational age. Crown–rump length—the measurement from the top of the head to the bottom of the torso—is commonly used during this period.

An early ultrasound may become particularly important when:

• the LMP is unknown or uncertain;
• cycles are irregular;
• hormonal contraception was recently stopped;
• breastfeeding affected menstruation;
• early bleeding makes the date hard to interpret;
• there is a large difference between menstrual and scan estimates; or
• knowing gestational age is clinically important for the care plan.

The scan still requires skilled acquisition and interpretation. A measurement is not meaningful in isolation from scan timing, image quality and clinical history.

Why might a doctor or midwife change the due date?

The LMP and ultrasound may point to different gestational ages. Clinical guidelines set discrepancy thresholds that depend on how far along the pregnancy is; the threshold generally becomes wider later because ultrasound dating becomes less precise.

Your care team may revise the official date when the early scan is sufficiently different from the menstrual estimate, especially if the LMP is unreliable. Ask four questions:

1. What date did we start with?
2. What measurement produced the new estimate?
3. How large was the difference?
4. Which due date is now documented as official?

Changing a date after a reliable first-trimester assessment should be uncommon. A later scan showing a smaller or larger fetus may raise a growth question rather than mean the pregnancy started on a different day.

Keep a single official date across referrals, calendars and family planning. Competing dates create avoidable confusion about screening windows and decisions.

How are IVF and known-conception pregnancies dated?

Assisted reproduction provides information that ordinary cycle tracking usually cannot. For IVF, ACOG recommends using the embryo's age and the date of embryo transfer to establish the due date.

A day-three embryo and a day-five embryo represent different points in development on transfer day, so a generic “conception date plus 38 weeks” entry can be wrong if it ignores embryo age. Use the date supplied by the fertility clinic.

Intrauterine insemination, ovulation induction and a positive ovulation test may narrow timing but do not always identify fertilization precisely. The care team decides how that information fits with ultrasound and cycle history.

Do not substitute the date of intercourse for the date of conception. Sperm may survive for several days, so the two are not necessarily the same.

Why does accurate gestational age matter?

Pregnancy dating affects much more than a countdown. The best obstetric estimate helps clinicians:

• schedule screening and diagnostic tests in valid time windows;
• interpret ultrasound anatomy and growth;
• distinguish preterm, term and post-term pregnancy;
• plan monitoring when pregnancy continues beyond the estimated date;
• interpret symptoms and clinical measurements; and
• communicate consistently across maternity services.

The World Health Organization recommendation on routine ultrasound supports one ultrasound before 24 weeks for pregnant people, including to estimate gestational age, improve detection of fetal anomalies and multiple pregnancy, and improve the pregnancy experience. Local schedules and individual needs vary.

This is why “my app says 12 weeks” should not replace “my clinical record says 11 weeks and 4 days.” The clinical estimate is attached to decisions that a consumer calculator cannot make.

What if you do not know the last menstrual period?

Not knowing is common and does not require inventing a date. Tell the maternity professional that the LMP is uncertain.

Bring information that may help:

• dates and results of pregnancy tests;
• the pattern of recent bleeding;
• usual cycle length and regularity;
• recent contraception or breastfeeding history;
• fertility treatment, ovulation or embryo-transfer records; and
• any previous ultrasound report.

The care team can decide which details are clinically useful and whether ultrasound dating is needed. Honest uncertainty is safer than a precise guess.

What if the calculator and scan disagree?

Keep both results long enough to ask the clinician, but use the documented clinical estimate after it is explained. A calculator knows only the dates entered. The clinician can assess menstrual reliability, scan quality, gestational age and relevant guidelines.

Do not repeatedly recalculate after every scan. Later ultrasound is often intended to assess anatomy, placental location, fluid or growth, not reset the first trimester.

If records show different official dates across providers, ask the maternity team to reconcile them and update referrals. ACOG emphasizes documenting the estimated due date and discussing subsequent changes.

Does the due date predict when labor starts?

No. The estimated due date organizes the pregnancy calendar; it does not forecast spontaneous labor for an individual.

ACOG's fetal-growth overview notes that only about 1 in 20 people give birth on the estimated date. Many births occur before or after it within a clinically expected range.

Avoid planning important travel, leave or support around one exact day. Build a flexible window and ask the care team when monitoring or intervention discussions would occur if pregnancy continues.

For the larger timeline, use the pregnancy week-by-week trimester guide. When the third trimester arrives, the practical hospital bag checklist helps turn the estimate into calm preparation.

What should you do with your due-date estimate?

1. Calculate once for orientation. Use the due date calculator with the best LMP information you have.
2. Start prenatal care. Share cycle history, medicines, conditions and fertility-treatment details.
3. Ask what established the official date. Was it LMP, early ultrasound, IVF timing or a combination?
4. Write the clinical date down. Use it for appointments and screening windows.
5. Ask before changing it. A later size estimate does not automatically mean a new due date.
6. Plan a birth window. Make leave, transport and support flexible around the estimate.
7. Keep both parents aligned. A shared Egspect family space can translate one date into role-specific weekly guidance.

Related reading and tools

• Free due date calculator
• Pregnancy week calculator
• Pregnancy week by week: trimester guide
• Ovulation and the fertile window explained
• Hospital bag checklist: what you actually need
• Pregnancy guide for fathers and partners
• ACOG: Methods for Estimating the Due Date
• WHO: ultrasound before 24 weeks

Clinical guidance was reviewed and accurate as of July 29, 2026. Professional recommendations and local maternity pathways can change.