At 20 weeks pregnant, your baby is about 6⅓ inches (160 millimeters) long from head to bottom and weighs roughly 11 ounces (320 grams). That’s about the length of a banana. This is the halfway point of pregnancy, and it’s often when you’ll have your detailed anatomy ultrasound, so size is very much on your mind.
Length, Weight, and How They’re Measured
The 6⅓-inch measurement is what’s called crown-to-rump length, meaning from the top of your baby’s head to their bottom, not including the legs. Earlier in pregnancy, this is the standard way babies are measured because their legs are curled tightly against their body. Around week 20, many providers begin switching to crown-to-heel measurements, which include the full length of the legs. When the legs are factored in, a 20-week baby measures closer to 10 inches from head to toe.
At 11 ounces, your baby weighs a little less than a can of soda. Weight gain accelerates significantly from this point forward. Over the next 20 weeks, your baby will multiply that weight roughly tenfold.
What the Anatomy Scan Measures
Most people have their mid-pregnancy ultrasound right around 20 weeks, and the sonographer takes several specific measurements to assess your baby’s growth. Three of the most important are the width of the skull, the circumference of the head, and the length of the thighbone.
At 20 weeks, the median skull width is about 46 millimeters, roughly the size of a golf ball. Head circumference sits around 171 to 172 millimeters, and thighbone length is about 32 millimeters. These numbers are remarkably consistent across different populations. NIH data shows the median values vary by less than a millimeter regardless of the mother’s racial or ethnic background, which means the “normal” range at this stage is narrow and well established.
Your provider plots these measurements on a growth chart and gives you a percentile. A baby at the 50th percentile is exactly average. Anywhere between the 10th and 90th percentile is considered normal. What matters most isn’t a single measurement but whether your baby tracks consistently along a growth curve over time.
What Your Baby Looks Like at 20 Weeks
By this point, your baby has moved well beyond a generic embryonic shape. Arms and legs are proportional enough to stretch, kick, and move freely in the amniotic fluid. The digestive system is functioning, meaning your baby is swallowing amniotic fluid, processing it through working kidneys, and passing it back out. This cycle helps develop the gut and keeps amniotic fluid levels balanced.
A greasy, cheese-like coating called vernix has started forming over the skin. This protective layer prevents the skin from getting waterlogged or irritated by constant exposure to amniotic fluid. Over the next week or so, a fine layer of downy hair will cover your baby’s entire body, helping the vernix stick in place. Both the coating and the hair are temporary and will largely disappear before birth.
Your baby now has regular sleep and wake cycles. They respond to sound and movement, so a loud noise or a sudden shift in your position can wake them up. If you haven’t felt movement yet, this is often the week it starts. Those first flutters, sometimes called quickening, feel like light tapping or bubbling. They’re easy to miss or mistake for gas at first, but they become unmistakable within a few weeks.
How Your Body Reflects the Growth
Your body gives its own clue about your baby’s size. By 20 weeks, the top of the uterus (called the fundus) reaches your belly button. From this point forward, there’s a handy rule of thumb: the distance in centimeters from your pubic bone to the top of the uterus roughly matches how many weeks pregnant you are. So at 20 weeks, you can expect the measurement to be close to 20 centimeters. Your provider will start checking this at prenatal visits as a simple, quick way to confirm that growth is on track between ultrasounds.
When Size Falls Outside the Average
Babies at 20 weeks don’t all measure exactly the same, and a number slightly above or below the average rarely signals a problem on its own. Genetics play a significant role. Taller parents tend to have longer babies, and smaller-framed parents tend to have smaller ones. The baby’s sex matters too; boys tend to measure slightly larger than girls at the same gestational age.
What raises concern is when a measurement falls well below the 10th percentile or well above the 90th, or when growth stalls between appointments. In those cases, your provider may schedule additional ultrasounds to monitor the trend. A single measurement is just a snapshot. The trajectory over several weeks tells the real story.