Your baby measures 6.9 in (17.5 cm) from crown to rump and weighs 12.7 oz (360 g).

Sebastian Kaulitzki / Adobe Stock

Pregnancy Facts - Week 21

How big is your baby in week 21 of pregnancy

This is where the world of baby sizing gets confusing.

How big is your baby in week 21 of pregnancy? 6.9 inches (17.5 centimeters).

You might be saying to your screen right now, "Hey Babysizer, my other pregnancy apps and sites are wicked smaht. They say my baby's like 4 inches (10.2 centimeters) bigger than what you're telling me. What's up?" They aren't wrong, they're just complicating things.

When Babysizer says your baby measures 6.9 inches, it is from crown to rump, which means from the top of your baby's head to their bottom. At week 21 of pregnancy, instead of measuring a baby from crown to rump, or crown-rump length (CRL), many other pregnancy resources switch to using crown to heel, or crown-heel length (CHL), which measures length from head to feet. That's why when you're using those resources your baby seemingly has a giant growth spurt between week 20 and 21.

So why do they switch measurement systems halfway through?


There's no good reason. It's the thing everyone does, so everyone continues doing it. While CHL is great for imagining your baby uncurled and out of the womb, your baby certainly didn't decide to plank in your uterus beginning this week. That would be... uncomfortable.


Using crown-rump length for every week of pregnancy makes more sense than switching halfway through. So that's what Babysizer does.

Luckily, everyone sensibly agrees that the weight scale should not change at week 21. So, this week, your baby weighs 12.7 ounces (360 grams or 0.0142 firkins or 1800 carats or 232 pennyweights or exactly one 21-week fetus).

Your baby's development in week 21 of pregnancy

Let's get back to weekly baby development. At week 21, all your baby's critical organs have now formed to varying degrees. While there is still a lot of growing to be done, the focus now for your baby is your primarily getting bigger and stronger. Having said that, this week's new development is mostly on them "down there" parts.

If your baby is male, his testicles will descend from his abdomen into his scrotum. Think Times Square ball drop on New Year's Eve or bungee jumping off a bridge.


If your baby is female, then this is the week her vagina begins to form. Think... vagina forming?

The only other major thing happening this week is that the skin covering your baby's eyes has now separated down the middle, forming eyelids. Your baby now has the power to blink, but it will be [a few more weeks before any blinking actually occurs](/facts/24].

My baby 6.9 in (17.5 cm) from crown to rump and weighs 12.7 oz (360 g)