Facts:Your baby's about 5.5 inches from head to toe, although when you go for your ultrasound the sonographer may measure from head to bum, as it's much easier due to the curled up position your baby is in. Your newborn will remain in this shrimp-like position for a few weeks after the birth, until they realise they have room to spread themselves out.
Weighing in at about 6.5 ounces, your baby is getting heavier by the day. The main activity at the moment is putting on fat, as most of the major development is now complete. Adding fat and putting on weight keeps your baby nourished, healthy and warm. Once he or she has enough fat, the fine hair covering the body will start to vanish, as it's no longer required for warmth. The fat is also helping the skin to become more opaque, although at this point the blood vessels inside your baby can still easily be seen - that's if you could peek into your uterus, of course.
At 18 weeks, the sulci in the brain begin to form. These are the little indents that give the human brain its characteristically bumpy look. This happens at the back of the brain first, and is quickly followed by the front. Your baby's brain is controlling more and more of the activities in the womb, so there's almost no random movements any more. Your baby can even yawn at nap times!
Both girls and boys have testosterone, but by 18 weeks the difference in these hormone levels between boys and girls is noticeable. Your baby boy's body will have testosterone racing round, while your baby girl will have much less, but will have her ovaries and fallopian tubes in place.