Due date by last period vs ultrasound: which one should you use?

Short answer: use the date your care provider gives you. It often starts from your last period and is then confirmed or adjusted by an early ultrasound.

1. The last-period method

Add 280 days to the first day of your last period. It is quick, but it assumes a 28-day cycle with ovulation around day 14.

2. The ultrasound method

An early scan measures the baby and estimates how far along you are. If it differs enough from your period date, the due date may be moved.

3. Why they differ

Longer or irregular cycles, an unsure period date, or simply normal measurement variation.

4. After the date changes

Update your calendar right away so your week, countdown and weekly information match what your care provider says.

Doing this with Ninemoon

Open Settings → Due date and pick “Weeks from an ultrasound”. Enter the scan date and the weeks and days you were told; Ninemoon recalculates the due date, your week and the calendar instantly.

FAQ

My cycles are irregular. Is the LMP due date wrong?

It can be off, because the 280-day rule assumes ovulation around day 14. That is one reason an early ultrasound is often used to confirm the date.

Can my due date change late in pregnancy?

Care providers usually keep the date set early in pregnancy. Ask yours before changing anything.

Get Ninemoon, the no-sign-up pregnancy calendar (Google Play)

← How Ninemoon shows your week, day and countdown at a glance