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.
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.
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.
- Idaho Department of Health and Welfare: Fetal Development
- Baby Center: What Your Baby Looks Like -- 13 Weeks
- WomensHealth.gov: Stages of Pregnancy
- U.S. Centers for Disease Control and Prevention: Fetal Development Chart
- Merck Manuals: Stages of Development of the Fetus
- Zero to Three: Prenatal Development
- Photo Credit herjua/iStock/Getty Images
Pregnancy Size by Week
Are you pregnant and wondering whether you are the "right" size for your week? While most good doctors will tell you that...
About the Fetal Heartbeat
A normal fetal heartbeat is one of the most reassuring sounds expectant parents will ever hear, as it normally indicates that all...
Stages of Baby Growth During Pregnancy
Before you know you're pregnant, your baby's sex has already been determined. And, by the time you're sporting a tiny baby bump,...
What Does a Baby Look Like at 3 Months in the Womb?
During pregnancy, a baby takes approximately nine months to grow from a fertilized egg into a little human being. Month by month,...
The Development of a Two Month Old Fetus
Your baby develops rapidly during the early months of pregnancy. A cluster of cells in the fourth week grows and develops to...
Development of a 13-Week-Old Baby
When your baby is 13 weeks, or about three months old, he will be experiencing a lot of new developments. You may...
Development of a Baby Fetus at 6 Months
If your baby's quarters are feeling a little more cramped, it's because they are. At six months pregnant, your baby is preparing...
How Big Is Your Baby at 12 Weeks in the Womb?
Twelve weeks marks a milestone for many pregnant women. It’s the end of the first trimester and often means fewer uncomfortable pregnancy...
How to Tell Gender on Ultrasounds
Fetuses of both genders start with androgynous genitalia that develops over time. The only way to determine sex during a sonogram is...