What Does a 13-Week-Old Fetus Look Like?

By Erica Loop
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The pink plus sign pops up on the pregnancy test, and now you’re wondering what’s going on with the little bundle of joy who is growing inside of you. By the end of your first trimester, your baby is developing recognizable features. If you want to paint a mental picture of what a 13-week-old fetus looks like, understanding the major milestones and markers of this time can help you to prep for what you might see on an ultrasound.

Head First

A 13-week-old fetus is starting to look more like what you may think of when you hear the word “baby.” That said, the fetus isn’t entirely in proportion. The head is the most noticeable feature at 13-weeks, according to the Idaho Department of Health and Welfare. Even though the head is developing into a more proportional part of the fetus, it’s still roughly one-third of the entire body size.

Height and Weight

Take a look at your pinky finger. At 13-weeks, your baby is about the same size as it. The fetus is slightly over 3 inches long at this point. Your baby is far from birth weight at 13 weeks. By the time you’re in week 37 of your pregnancy, it’s likely that your baby will be at least 6 pounds.The 13-week fetus weighs in at slightly more than 1 1/2 ounces.

Organs and Control

Even though your baby won't need a diaper for months, her kidneys are making urine by 13 weeks. By this time, the external genitals are developed, according to the U.S. Centers for Disease Control and Prevention. Your baby’s brain continues to grow exponentially by the start of the second trimester. By the time you reach the end of the first trimester your baby is able to coordinate basic actions such as thumb-sucking. At 13 weeks, fetal brain development includes even more complex activities such as rhythmic breathing and the swallowing reflex.

Facial Features

Don’t expect to see open eyes during an ultrasound yet. The 13-week-old fetus’s eyelids are closed to protect your developing baby’s eyes until they open somewhere around week 28. Your fetus doesn’t have teeth yet but does have about 20 buds that turn into baby teeth. The nose is forming, with the bridge visible by 13 weeks.

Muscles and Movement

Whether you’re watching your baby on an ultrasound or you have your hand on your belly, it’s likely you know that a 13-week fetus is capable of movement. You might feel light flutterings of movement, although more pronounced movements typically don't begin until week 16 or later. By this point in your pregnancy, the baby has joints and muscles that let her move. Even though your baby is on the move, the skeleton is still incomplete and continuing to develop.

About the Author

Based in Pittsburgh, Erica Loop has been writing education, child development and parenting articles since 2009. Her articles have appeared in "Pittsburgh Parent Magazine" and the website PBS Parents. She has a Master of Science in applied developmental psychology from the University of Pittsburgh's School of Education.