Fetal development in pregnancy week 15:fetus in fourth month Your nearly four-inch long gymnast is happily mobile inside your womb and if you're really lucky, you’ll notice a point when your sneezing, coughing or laughing results in a little kick here or a poke there. Still, many women don't feel anything until the 17th week or later. Although the poking and kicking isn't very charming during sleeping hours, it’s a good sign as it means your baby is actually reacting to outside events. Yay! You’ve got yourself one active healthy baby! Their little elbows and knees are bending more freely this week and their little legs are finally growing longer than their arms and getting pumped up for prenatal Richard Simmons. Many of their major organ systems are increasing in capacity—particularly that amazing tiny heart and complex circulation system, which is now pumping about 25 quarts of blood per day, and will increase to a very impressive 200 quarts per day by the end of the pregnancy. As far as hair goes, we’ve got some new scalp patterns beginning to develop on the head, although actual head hair is not yet present.