How does an ultrasound detect when your last period was?

Answer 1

An ultrasound does not detect when your last period was. An ultrasound measures the size of your baby compared with the millions of others that have been scanned and gives an estimated date of delivery. Up to about 20 weeks of pregnancy all babies are about the same size. After that the size of the baby depends more and more on genetics and nutrition. Counting back 40 weeks from the estimated date of delivery gives an indication of when the last period might have been.

Answer 2

They can guess by looking at an ultrasound. Your period is a good indicator as well. But it's all guess work really. But they're usually pretty good at it.

Both. If your periods are regular every 28 days your due date is pretty close to 40 weeks after your last one. However if your periods are irregular and/or you can't remember the ultrasound is accurate to witin 2 or 3 days up until about 8 weeks after conception as embryos all grow at the same rate.

After about 16 weeks it gets less accurate as some grow faster than others.

Ultrasounds and calculations from last period are a matter of statistics NOT guesswork

Answer 3

As long as you are very early in pregnancy, a doctor can usually tell you from looking at an ultrasound. There is very little variability in size very early on. It is later in pregnancy when individual differences arise.