Can Your Due Date Change?

As we explain in how due dates are calculated, the standard method (Naegele's Rule) adds 280 days to the first day of your last menstrual period and assumes a textbook 28-day cycle with ovulation on day 14. That assumption doesn't hold for everyone.

If your cycles run longer or shorter than 28 days, if you're not entirely sure when your last period started, or if you conceived while breastfeeding or shortly after stopping hormonal birth control (both of which can make cycles irregular for a while), the calendar-based due date can be meaningfully off from your baby's actual gestational age.

The most common reason a due date changes is a first-trimester ultrasound. Between roughly 8 and 13 weeks, fetal growth is remarkably consistent from one pregnancy to the next, which means measurements like crown-rump length can estimate gestational age quite precisely — generally more precisely than LMP-based calendar math alone, according to ACOG.

If the ultrasound estimate and the LMP-based estimate land close together, your provider will typically leave your due date as originally calculated. If they diverge by more than a small, clinically meaningful amount, your provider may update your due date to match the ultrasound. This is especially common when your periods are irregular, you don't recall your LMP precisely, or you conceived via fertility treatment with a known transfer or ovulation date that gives an even more exact starting point.

Our Due Date Calculator is built around the LMP/conception-date method, which makes it a great first estimate — but your ultrasound dating, if it differs, is what your medical team will typically go by from that point on.

You might wonder why a 30-week ultrasound showing your baby measuring "a week ahead" or "a week behind" doesn't trigger another due date change. The reason is that early in pregnancy, nearly all babies grow at a very similar rate, which makes early measurements a reliable proxy for age. Later in pregnancy, healthy babies naturally grow at different rates — some are simply going to be bigger or smaller than average — so size is no longer a reliable way to pin down gestational age.

That's why providers generally lock in the due date using the most reliable early estimate (LMP adjusted for cycle length, or an early ultrasound) and then use later ultrasounds to monitor growth and wellbeing, not to re-date the pregnancy.

• It doesn't mean your baby is growing too fast or too slow.
• It doesn't change how far along you actually are — it corrects the estimate to better match reality.
• It can shift which trimester-based milestones and screening windows apply to you.
• It's far more likely with irregular cycles, uncertain LMP, or conception shortly after stopping birth control.

If your due date is revised, it's worth updating any tracking you're doing — apps, calculators, or your own calendar — to the new date, since weekly milestones and screening recommendations are typically tied to gestational age, not the calendar date you originally expected.

It's common to end up with more than one number floating around — the date an app gave you, the date a calculator gave you, and the date your provider wrote on your chart. When they don't match, the one that matters for medical decisions is whatever your prenatal provider currently has on file, since that's the number used to time screening tests, growth checks, and decisions around induction if your pregnancy goes past term.

A few situations make it especially worth double-checking with your provider rather than relying on a calculator alone: if you're not sure of your LMP date, if your cycles have been irregular for months before conceiving, if you conceived shortly after stopping hormonal birth control, or if you've had more than one due date communicated to you at different appointments. In any of those cases, ask directly which date is currently being used — it's a completely normal question, and your provider would rather clarify it than have you tracking the wrong week.

Outside of those situations, it's reasonable to treat your due date the way most providers do: a useful planning target that's allowed to be off by a week or two in either direction, with the real answer arriving whenever your baby decides to.

Yes, your due date can change — and for most people, if it does, it's because an early ultrasound gave a more precise estimate than the original calendar math. It's a routine part of prenatal care, not a cause for concern. If you're ever unsure which due date to go by, your prenatal provider's most recent estimate is always the one to trust.