Week 20 is about the time when most moms can feel the baby moving for the first time, and with good reason. At this point, the fetus weighs about 10 ounces and measures about 6 ½ inches from the top of his head to the bottom of his rump, and is almost 11 inches from head to heel. Babies are usually measured from head to bottom for the first 20 weeks because of the curled position can make head-to-toe measurements difficult; after 20 weeks, it is easier to measure a baby's full length as he stretches and makes better use of the uterus with increased movement. This increase in activity and his larger size are what allow you to feel his womb acrobatics.
Sometime between 18 and 20 weeks, you will probably have an ultrasound to check on the baby's progress and ensure things or going as expected inside the uterus. In addition to being an opportunity to lay eyes on your little one, this inside view can also tell you if you are having a boy or a girl if your baby is cooperating. Some babies will be facing the wrong direction or using their hands and legs to cover up their genital region. If you want the gender to remain a mystery, you should tell the technician and your caregiver early and often to prevent any unintentional information from slipping out.
Boy fetus testicles have begun their descent, but they are still in the abdomen as the scrotum isn't done growing. The scrotum will finish growing in a few more weeks, and the testicles will move down when it is ready. Girl fetuses will have a fully formed uterus, and the vaginal canal is starting to develop. She will also have about 7 million eggs in her tiny ovaries. By the time she is born, there will only be around 2 million, all she will ever have and more than she will ever need. For both female and male fetuses, the external genitalia is formed enough for a clear view of the parts of girls or boys.
No matter what sex the fetus is, they will have perfectly formed external ears at this point. Fetuses can hear your body sounds, including heartbeat and gastrointestinal digestion. He can also hear loud noises from the outside world and might react to them by moving away or jolting if startled. This makes week 20 a great time to start bonding with your baby from outside the womb with music and sounds they'll experience when they get to the outside world.
- American Pregnancy Association - Week 20
- American College of Gynecology and Obstetrics - How Your Fetus Grows During Pregnancy