Why am I 33 weeks pregnant but my ultrasound shows 29 weeks?

Question

I am currently 33 weeks pregnant, but my ultrasound is showing that I am only 29 weeks. What could be the reason for this discrepancy?

Answer 1

I know how confusing that can feel! Dating can get a bit messy because there are two ways they figure out how far along you are – based on the first day of your last period and based on how bub measures on the scan. If your cycles were a bit irregular or you ovulated later than “normal,” the LMP calculation can be off by a couple of weeks. 

Later in pregnancy, ultrasounds are also less precise at estimating weeks because babies grow at their own pace. Some are little peanuts and measure behind but catch up later, others are just genetically smaller. With my daughter I measured 2–3 weeks behind for most of the third trimester and my midwife wasn’t worried at all because the growth curve stayed steady and all the fluid/heartbeat checks were fine. She was born petite but healthy. 

It’s always worth chatting with your care provider to understand what they’re seeing. They might want to double check dates or keep an extra eye on growth just to be safe. But often it’s just one of those quirks of pregnancy math and not a cause for alarm. Hang in there!