The baby can hear mom’s heartbeat!
The baby is now able to hear their first sounds, such as the mother’s heartbeat, but they are not yet able to hear sounds from outside [1]. Their ears will be completely formed between weeks 22 and 24.
The brain is developing intensely this week. The left and right hemispheres are covered with grooves and folds, and the process of nerve cell division begins this week.
The baby’s arms and legs continue to grow and fingernails appear. Subcutaneous fatty tissue has not yet developed, so blood vessels can be seen through the layers of the skin, making the baby appear red.
This week, they also start growing hair! Soon their hair color will be determined by their genes and the cells responsible for the color will begin to produce it.
If your partner is expecting twins
An ultrasound may find that one of the babies is ahead of the other in height [2]. If they are dizygotic (derived from different eggs and different sperm), then there is no reason to worry. Especially if it turns out that one of the babies is a boy, and the other a girl (which can likely be determined at this time) [3]. The difference in size is considered completely normal in these cases.
But if the twins are monozygotic (identical) and have a common placenta, then doctors will carefully monitor whether there is delayed growth in one of the babies. Therefore the expectant mother may be asked to come in for ultrasounds more often [2].
What we can see on an ultrasound
Bone marrow development continues and internal organs begin to work. The intestine is gradually filled with meconium, the baby’s first poop, which is a greenish-black color because of its bile content.
For development and growth, the baby needs an increased blood supply and as many nutrients as possible. The load on the placenta increases, and it grows to be a little over half an inch (17.7 mm) thick.
