How’s My Baby Growing? Fetal Development From 13-16 Weeks

16-wks-nature-momWelcome to the second trimester—a period of rapid growth for both you and Baby. Here’s the weekly guide to what’s happening in utero. Note that these are general guidelines for the average fetus, and every baby’s rate of growth is different.

Week 13 

Baby measures nearly three inches long head to rump and weighs about one ounce. (like a plum). Even at this tiny size, he already has fingerprints. His intestines, which were partly located inside the umbilical cord) now migrate to their final resting place inside his abdomen, and ribs begin to form. Veins and organs are visible through his thin, transparent skin. His body begins to produce insulin, the hormone responsible for regulating blood sugar levels.

Due to changes in hormones (and increased blood flow and sensitivity), many women experience a lift in their libidos around this time. Take advantage of that with a romantic weekend getaway.

Week 14 

Baby measures 3.5 inches head to rump and weighs about 1.5 ounces (like a lemon). The liver, spleen and kidneys are in working order as they produce, respectively, bile, red blood cells and urine, which is released into the amniotic fluid. Baby’s body is growing at a faster rate to catch up with her ginormous head. Her eyes begin to move toward the front of the face, and the developing ears move from the neck to the sides of her head. Her nose, cheekbones and neck become more pronounced.

You’re liable to see your appetite return now if you were overcome with nausea or sickness during the first trimester. Fuel it with healthy, nutrient-rich foods for you and Baby.

Week 15

Baby measures four inches head to rump and weighs 2.5 ounces (like an apple). Although his bones and marrow will continue to develop, his joints and limbs can move now, and he may begin grasping and making a fist. Baby’s legs have grown longer than his arms. The fingernails and toenails are growing in. Even though his eyelids are fused shut, he can exhibit a repertoire of facial expressions from squinting to frowning. By the end of this week, the roof of his mouth will be fully formed.

If you’ve decided to have genetic or chromosomal testing done, you’ll undergo AFP (Alphafetoprotein) testing or an amniocentesis sometime between now and week 18. Educate yourself beforehand on whether prenatal testing is right for you.

Week 16

Baby measures about 4.5 inches head to rump and weighs 3.5 ounces (like an avocado). She’s growing eyelashes and eyebrows and tiny bones in her ears. She continues to practice breathing, inhaling and exhaling amniotic fluid in a movement that helps the lungs to grow. Her heart pumps 25 quarts of blood each day, and increased muscle growth has her working to lift her head from its bent-over position.

You may feel Baby’s first movements right about now—it usually feels like a sensation of flutters or gas bubbles—or you may not experience it for another few weeks. Take some time now to learn or brush up on infant CPR.

What’s next? Read about your baby’s fetal development from 17-20 weeks.

Got questions or ideas for future blog posts? Leave a comment below!

Donna DeForbes is a writer, designer and the force behind, a blog that explores ways to make going green fun and easy for the whole family. She is the author of the e-book, “The Guilt-Free Guide to Greening Your Holidays.” Follow her on Facebook and Twitter.

MammaBaby video Are you looking for a baby logging App? MammaBaby is a fully-synchronized App-Web environment to enable “meaningful use” of baby logs. Here’s how you can use MammaBaby to improve parenting experiences.
» Get the app » Sign Up for Free


  1. How’s My Baby Growing? Fetal Development From 9-12 Weeks | Life'n Stats - July 8, 2013

    […] What’s next? Read about your baby’s fetal development from 13-16 weeks. […]

Leave a Reply

Please support the site
Like uS on FB and receive one essential parenting tip weekly!