Due Date by Ultrasound Calculator
Had a dating scan? Enter the date and the gestational age your sonographer measured to find your due date and how far along you are today.
Keep your dates in Prego: a daily countdown, your baby's size each week and a checklist for every trimester.
Why an ultrasound can change your due date
A due date from your last period assumes you ovulated on day 14, which isn't true for everyone. An early ultrasound measures your baby's crown-rump length, which is very consistent between babies in the first trimester, so it dates the pregnancy more accurately.
ACOG recommends using the first-trimester ultrasound date if it differs from your period date by more than 5 to 7 days, depending on how early the scan was. Later scans are less accurate for dating because babies grow at different rates.
Frequently asked questions
Which is more accurate, ultrasound or LMP?
A first-trimester ultrasound is the most accurate way to date a pregnancy, especially if your cycles are irregular or you aren't sure of your last period.
Why did my due date change after the scan?
If your baby measured a few days or more ahead or behind your period-based dates, your provider may move your due date to match. It usually means you ovulated earlier or later than day 14, not that anything is wrong.
Will my due date change again at later scans?
Usually not. Once a pregnancy is dated by an early scan, providers keep that date. Later differences in size are tracked as growth, not as a new due date.
Estimates only. Your doctor or midwife may give you different dates or targets.