Can My Due Date Change?

Medically reviewed by: Safi Allah Khan · Written by the Momi team · Last updated 20 October 2026

This guide covers when a due date changes, ACOG's redating rules, why later scans rarely move it, and why the due date is really a window. To estimate yours, start with the due date calculator.

Dating scan vs your last period

Your first due date usually comes from the first day of your last period plus 280 days. Our guide to how is due date calculated explains the formula. That date assumes you ovulated around day 14 of a 28-day cycle, which isn't true for everyone.

An early ultrasound measures your baby from crown to rump. In the first trimester, babies grow at a very similar rate, so that measurement is a reliable guide to how far along you are. The NHS offers a dating scan at around 10 to 14 weeks. ACOG calls a first-trimester ultrasound, up to 13 weeks and 6 days, the most accurate way to establish or confirm gestational age.

When the two dates disagree by more than a set margin, the scan wins.

ACOG's redating rules

When the scan is done	Change the due date if the scan and LMP differ by more than
Up to 8 weeks 6 days	5 days
9 weeks 0 days to 13 weeks 6 days	7 days
14 weeks 0 days to 15 weeks 6 days	7 days
16 weeks 0 days to 21 weeks 6 days	10 days
22 weeks 0 days to 27 weeks 6 days	14 days
28 weeks and later	21 days

Source: ACOG Committee Opinion 700 (reaffirmed 2025). If the difference is smaller than these margins, your period-based date stays. If you conceived through IVF, the date from your embryo transfer is used and isn't changed by scans.

Why it usually changes only once

ACOG's guidance is that once the due date is set from the LMP and the first accurate scan, "subsequent changes to the EDD should be reserved for rare circumstances". There are good reasons:

• Early scans are the most precise. Later in pregnancy, healthy babies vary much more in size because of genetics, the placenta and other factors.
• Screening depends on it. Blood test results for conditions such as Down's syndrome are interpreted against gestational age, so the date needs to be right early.
• Decisions depend on it. Whether a baby is preterm, when to offer induction, and when you're "overdue" all rely on a stable date.

Can my due date change at the 20-week scan?

Usually no, if you've already had a dating scan. The NHS 20-week scan looks at your baby's anatomy, the placenta and blood flow. It's a check for 11 physical conditions, not a dating scan.

If your baby measures a little ahead or behind at 20 weeks, that's normally within the range of healthy variation. Your sonographer or midwife will tell you if anything needs follow-up.

The exception is if the 20-week scan is your first scan. Then it may be used to date the pregnancy. Between 16 and 21 weeks 6 days, ACOG advises changing the date only if the scan differs from your period date by more than 10 days. ACOG also says a pregnancy without a scan confirming the due date before 22 weeks should be considered "suboptimally dated".

Can my due date change in the third trimester?

Rarely. By 28 to 32 weeks, a measurement that's "ahead" or "behind" is much more likely to reflect your baby's size than a wrong date. Instead of moving the due date, your provider may:

• book growth scans
• measure your bump (fundal height) more often
• check the baby's movements, the fluid around them and blood flow

If someone suggests changing your date late in pregnancy, it's fair to ask why, and whether it's a dating question or a growth question.

What a "due window" means

The NHS says only about 1 in 20 babies arrive on their due date. Most come between 37 and 42 weeks:

Weeks	ACOG term
37 weeks 0 days to 38 weeks 6 days	Early term
39 weeks 0 days to 40 weeks 6 days	Full term
41 weeks 0 days to 41 weeks 6 days	Late term
42 weeks and beyond	Post-term

So rather than "my due date moved", it can help to think "my window is roughly these five weeks". The NHS suggests having your hospital bag packed at least 3 weeks before your due date. ACOG explains that if you pass your due date, your provider may start extra monitoring, and labor induction may be discussed from 41 weeks.

Reasons your date might be different from an app

If your app and your notes don't match, it's usually one of these:

1. You haven't updated the app after your dating scan.
2. Your cycle isn't 28 days. Longer cycles usually mean later ovulation and a later scan date.
3. You weren't sure of your period date, or had a period-like bleed in early pregnancy.
4. You recently stopped hormonal contraception or were breastfeeding, both of which can make cycles unpredictable.
5. Rounding. Some calculators count the first day of your period as day 0, others as day 1.

Our guide to how accurate pregnancy calculators are goes into more detail.

What to do if your date changes

• Use the new date everywhere: your notes, your app and any planning.
• Recheck your week with How Many Weeks Pregnant Am I?.
• Look at your appointment plan again. Screening and later checks are timed by week; see the pregnancy appointment schedule.
• Ask questions. It's reasonable to ask how much the date moved and why.

Keep your due date up to date in Momi

Momi sets your due date from your last period or conception date and follows your pregnancy week by week. After your dating scan, update the date so your progress ring, baby size and weekly updates match your notes. You can save your scan photos, log appointments and add medicine reminders in the same app.

Download Momi free on the App Store or Google Play. Free with ads; optional Premium.

Related: 12 weeks pregnant · 20 weeks pregnant · IVF due date calculator

Sources

• ACOG, Committee Opinion 700: Methods for Estimating the Due Date
• NHS, 12-week scan
• NHS, 20-week scan
• NHS Best Start in Life, Hospital bag checklist
• ACOG, When Pregnancy Goes Past Your Due Date

Medical disclaimer: Momi and this website provide general information, not medical advice. Always talk to your doctor or midwife; in an emergency call your local emergency number.