Due Date Calculator

Your due date is the anchor for nearly everything in prenatal care, from when to book scans to when screening tests are offered. The calculator above estimates it the same way clinicians do before an ultrasound. Below is the arithmetic behind each method, how IVF pregnancies are dated, and why your date may change.

How the due date calculator works

Pick the date you know best. The calculator adds a fixed number of days:

It then works backwards to a period-equivalent start date to show how many weeks and days along you are, and which trimester you are in.

Naegele's rule and the 280-day convention

By convention, the estimated due date is 280 days, or 40 weeks, after the first day of your last menstrual period.¹ The shortcut most clinicians learn is Naegele's rule: take the first day of your last period, add seven days, subtract three months and add one year. A last period starting January 1 gives a due date of October 8. The rule is named after the German obstetrician Franz Naegele, and a 2000 review argued it may have been misread over time in a way that moved due dates earlier.²

ACOG points out the limits. The 280-day rule assumes a regular 28-day cycle with ovulation on day 14, so it does not account for longer or shorter cycles, variable ovulation, or inaccurate recall of the last period.¹ About half of women accurately recall the date of their last period.¹

Adjusting for cycle length

If your cycles are longer than 28 days, you probably ovulate later, so the calculator moves your due date later by the same number of days, and earlier for shorter cycles. That follows from how cycles work: in an analysis of 612,613 cycles, differences in cycle length came mainly from the phase before ovulation, while the phase after ovulation stayed comparatively steady.³ If you tracked ovulation and know your conception date, choosing the conception option is more precise.

Due dates after IVF

For pregnancies from assisted reproduction, ACOG says the due date should be set from the IVF timeline rather than the last period, using the age of the embryo and the transfer date.¹ For a day-5 embryo, the due date is 261 days after the transfer; for a day-3 embryo, 263 days after.¹ Both land on the same point as 266 days after fertilization. Your clinic's date takes priority if it differs.

How accurate is your due date?

A due date is an estimate of the middle of a range, not a prediction of the day. In 125 naturally conceived pregnancies where ovulation was measured with daily urine hormones, the median time from ovulation to birth was 268 days, and lengths still varied by 37 days after excluding preterm births.⁴ Dating by last period was more variable than dating by ovulation, reflecting differences in when people ovulate.⁴

A first-trimester ultrasound is the most accurate dating method. Up to 13 weeks and 6 days, measuring the embryo's crown-rump length dates a pregnancy to within about 5 to 7 days.¹ ACOG advises changing the due date to match the ultrasound when it differs from period-based dating by:¹

Once set, ACOG says later changes to the due date should be rare.¹ If there was no ultrasound confirming or revising the date before 22 weeks, the pregnancy is considered suboptimally dated.¹

How far along are you?

Pregnancy weeks are counted from the first day of your last period, not from conception. In a 28-day cycle, that means you are already about two weeks pregnant on the day you conceive, and the 40-week count includes those two weeks.¹ That is why the calculator's conception estimate sits about two weeks after the period-equivalent date.

What to do next

• Book your first prenatal visit: our first prenatal appointment guide covers timing and what to bring.
• Keep taking a prenatal with folic acid; see prenatal nutrient doses for what to look for on the label.
• Work through the first trimester checklist, and see which early pregnancy symptoms are normal.

If you are not sure yet, the DPO calculator shows when a pregnancy test is reliable. For everything in this step, go to the Pregnant hub.