What you can't fully expect when you're expecting: Accurate gender

An obstetric ultrasound of a fetus at the fourth month.

Story highlights

  • Depending on who is evaluating the ultrasound, they can be about 90% effective
  • Condoms are one of the least effective birth control methods, with a failure rate of 18%

(CNN)For expecting parents, 20 weeks is a milestone. It marks the halfway point of the 40 week pregnancy, but most excitingly it is the time many soon-to-be moms and dads find out the answer to the big question: boy or girl?

But what happens when instead of the little girl everyone has been expecting and buying clothes for a little boy ends up being born? Kyle and Danielle Williams captured their family's reaction on camera, and their reactions went viral.
How likely is this mix up? "It depends on how the baby is situated in the uterus," said sonogr