How Your Due Date Is Calculated (and Why It Can Change)

The short answer

Your estimated due date is 280 days (40 weeks) after the first day of your last menstrual period. That is the convention described by the American College of Obstetricians and Gynecologists in Committee Opinion 700. If your last period started on February 2, 2026, your due date is November 9, 2026.

That first date is often adjusted. ACOG says an ultrasound in the first trimester, up to and including 13 weeks 6 days, is the most accurate way to set or confirm how far along you are. If the scan and your period date disagree by more than a set number of days, your provider will usually move the due date to match the scan. The due date calculator shows both, and the rest of this guide explains why.

Why it starts from your period, not conception

Pregnancy is counted from the first day of the last period, about two weeks before conception in a textbook cycle. So on the day you conceive, you are already counted as about 2 weeks pregnant. ACOG notes the 280-day rule assumes a regular 28-day cycle with ovulation on day 14. It doesn’t account for:

• Memory. ACOG cites research that only about half of women accurately recall their last period.
• Cycle length. Not everyone has a 28-day cycle.
• Ovulation timing. It varies, even in the same person.

That is why the period-based date is a first estimate, not the final word.

The four ways to work it out

Starting point	Due date	Where it comes from
First day of last period	+ 280 days	ACOG convention
Conception date	+ 266 days	280 days minus the 14 days before ovulation
IVF, day-5 embryo transfer	+ 261 days	ACOG Committee Opinion 700
IVF, day-3 embryo transfer	+ 263 days	ACOG Committee Opinion 700
First-trimester ultrasound	Scan date + 280 days − age at the scan	Scan dating, which ACOG ranks most accurate

For IVF, ACOG says the due date should be assigned from the age of the embryo and the date of transfer, not from a period.

Worked example: four routes to the same date

All of these land on November 9, 2026:

• Last period February 2, 2026: + 280 days.
• Conception February 16, 2026: + 266 days.
• Day-5 embryo transferred February 21, 2026: + 261 days.
• Day-3 embryo transferred February 19, 2026: + 263 days.

On October 12, 2026, there would be 28 days to go, so 280 − 28 = 252 days, or 36 weeks 0 days.

What about a longer or shorter cycle?

A common clinical rule of thumb shifts the date by the difference from 28 days. A 32-day cycle adds 4 days, moving the example to November 13; a 25-day cycle takes off 3. This adjustment is not part of ACOG’s guidance, and an early ultrasound settles the question either way. Leave the cycle at 28 in the calculator for the plain 280-day date.

When an ultrasound changes your due date

In the first trimester, the crown-rump length of the embryo (head to bottom) dates a pregnancy to within about 5 to 7 days, according to ACOG. Later scans measure the head, abdomen and thigh bone, and get less precise as the pregnancy goes on. So the gap needed to change the date grows with time.

ACOG’s Table 1 thresholds, by your age from your last period on the day of the scan:

When the scan is done	Change the date if it differs by more than
Up to 8 weeks 6 days	5 days
9 weeks to 13 weeks 6 days	7 days
14 weeks to 15 weeks 6 days	7 days
16 weeks to 21 weeks 6 days	10 days
22 weeks to 27 weeks 6 days	14 days
28 weeks and later	21 days

ACOG gives a sense of how often this happens: in one study it cites, 40% of women who had a first-trimester ultrasound had their due date adjusted because the scan and the period date differed by more than 5 days.

Worked example: a scan that moves the date

Her last period started February 2, 2026, giving November 9. She has a scan on April 6, when she is 9 weeks 0 days by her period. The scan measures the embryo at 7 weeks 4 days. That is a 10-day difference, more than the 7-day threshold for 9 to 13 weeks 6 days. So her provider would usually change the due date to the scan date: April 6 + 280 days − 53 days = November 19, 2026.

If the scan had measured 8 weeks 3 days, the gap would be 4 days, and the period date would usually stand.

Third-trimester scans

ACOG calls ultrasound dating at 28 weeks and later the least reliable method, accurate to about 21 to 30 days. A baby that measures small may be growing slowly rather than younger than thought, so moving the date late in pregnancy needs care and the whole clinical picture.

Why your date shouldn’t keep changing

ACOG recommends that once your provider has your period date, your first accurate ultrasound or both, the due date should be set, discussed with you and recorded. After that, changes should be reserved for rare circumstances. A pregnancy without an ultrasound that confirms or revises the date before 22 weeks is considered “suboptimally dated.” If you get different dates from different apps or scans, ask your provider which one is in your record.

Your due date is the middle of a window

A due date marks 40 weeks 0 days. ACOG and the Society for Maternal-Fetal Medicine, as summarized by the NICHD, define:

• Early term: 37 weeks to 38 weeks 6 days
• Full term: 39 weeks to 40 weeks 6 days
• Late term: 41 weeks to 41 weeks 6 days
• Post-term: 42 weeks and beyond

The due date sits in the middle of full term, not at the end of it.

Trimesters

Using ACOG’s ultrasound ranges: the first trimester runs up to 13 weeks 6 days, the second from 14 weeks to 27 weeks 6 days, and the third from 28 weeks onward. The calculator shows which one you are in today.

Planning around the date

• Food. The Dietary Guidelines estimate no extra calories in the first trimester, about 340 more a day in the second and about 452 in the third, for women with a healthy weight before pregnancy. They advise following your healthcare provider’s guidance. The food calorie lookup shows what those amounts look like in real portions.
• Activity. The Physical Activity Guidelines say women should get at least 150 minutes of moderate aerobic activity a week during pregnancy and after birth, under the care of a health care provider. If walking is your activity, the steps to miles calculator turns a step count into distance.

Common mistakes

• Counting from conception and adding 280. That puts the date two weeks late. From conception, add 266.
• Assuming the first date is final. An early scan often refines it.
• Using a period date after IVF. Use the transfer date and embryo age.
• Treating the due date as a deadline. Birth anywhere in the term window is normal.

This guide explains how due dates are estimated and is not medical advice. Your doctor or midwife sets your due date. If you have bleeding, pain or any concern during pregnancy, call them right away.