In Depth Baby Product Reviews Led by a Pediatrician

Big Things and Brain Folds

Week 13 sees your little one's brain growing and details forming
Big Things and Brain Folds
Week 13 fetus
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Sunday September 29, 2019

Your fetus is about 3 inches long and weighs approximately 1 ounce, making him about the size of a small peach or lemon. His head is about a third of the size of his body and is starting to look more balanced as his body straightens and his back begins flexing.

His eyes are still fused shut as they continue to develop and his toenails and fingernails will start developing in earnest over the next few weeks. The vocal cords are also shaping up this week, giving life to his ability to cry and coo once he makes his big arrival. While the fetus' sex organs are now fully formed, they are still too small to see well on ultrasound, and any guess your provider makes at this point would only be a guess. It is best to wait until after 18 weeks before verifying the baby's gender on the ultrasound and planning your gender reveal party. However, if you choose to have non-invasive prenatal testing (NIPT) for chromosomal abnormalities, you may already know the gender.

Your fetus's brain begins to develop the grooves and folds known as sulci. These folds are an integral part of the brain and increase the overall surface area of the brain, providing more square footage for the nervous system to do its thing.

Reference Sources

  1. American Association of Pregnancy - Pregnancy Week 13
  1. WebMD - Your Pregnancy Week by Week: Weeks 13-16
  1. Mayo Clinic - Fetal development: The 2nd trimester