Week 3 to 4
First heartbeat and neural tube development
- PCW 4iPost-Conception Week
- Days 21-27
- Gestational Week 6
The changes in the embryo between the beginning and end of the fourth week are dramatic. Like rumpling a flattened blanket, flattened embryonic tissue begins folding, forming a tube that will become the brain and spinal cord. More folding forms the chest and abdominal cavities. Clusters of cells start growing in just the right locations to form the upper and lower limb buds. These buds will become the arms and legs.4
Furthermore, after only 22 days in the womb, the embryo has a heartbeat!5 At this point, a woman will have likely been aware of her pregnancy for only about a week.
Doctors use a variety of methods to listen to the fetal heartbeat. At first, the sound and motion from the heart is barely detectable. Doctors use ultrasound technology to see the motion of the heart in early pregnancy. Starting 10 weeks after conception, many doctors will use a Doppler fetal monitor to check the fetus’s heart rate.8 A fetal Doppler monitor uses the motion of the blood within the heart to make an audible signal and measure the heart rate.
Just 22 days after fertilization, the neural tube starts to close. Folds of neural tissue fuse together, starting in the neck area, continuing towards the head and rump. The neural tube becomes the brain and spinal cord. The brain finishes fusing by the 25th day, and the bottom of the spinal cord finishes fusing by the 28th day. The brain grows in three sections: the forebrain, midbrain, and hindbrain.9 The forebrain is responsible for sensing, decision making, and forming memories; the midbrain is responsible for localizing sounds, moving, and tracking objects; and the hindbrain is responsible for vital body functions. Later in development, pain processing occurs in all three sections of the brain.10
Doctors can count the number of small bumps along an embryo’s back to accurately estimate her age. The bumps are called somites and first appear 20 days after conception. The number of somites present in the embryo in the following week can be used to give an exact age. For example, at 22 days the embryo has 7 distinct somites, while by day 28 there are about 25 somites. As somites mature, they form bones and muscles.11