Your developing baby – week 13

by Kaiser Permanente |
Fetus at week 13, illustration

At 13 weeks, your baby is about 3.5 inches long around the size of a plum. By the time you reach week 13, the placenta has fully formed and begun providing your baby with nutrients and oxygen. It also removes the baby’s waste. 

Your baby’s:

  • Veins and internal organs appear through the baby’s nearly see-through (transparent) skin.
  • Muscles and bones continue to develop and strengthen.
  • Arms and legs are long and thin. Fingerprints have formed on the baby's tiny fingers.
  • Hearing starts to develop.

Around week 13, your baby’s able to:

  • Make a fist with each hand.
  • Put a thumb in the mouth and make a sucking motion.

Your baby has more muscle tissue and harder, more developed bones. Your little one is starting to roll, kick, and move around a lot — flexing tiny arms and legs, too. You may be able to start feeling these movements around week 18.

This article has been created by a national group of Kaiser Permanente ob-gyns, certified nurse-midwives, pediatricians, lactation consultants and other specialists who came together to provide you with the best pregnancy, birth, postpartum, and newborn information.

Some of the content is used and adapted with permission of The Permanente Medical Group.