A fertilized human egg is less than 0.1 mm across. A full-term baby averages a little over 19 inches in length. This remarkable growth occurs over the course of nine months. During the last four weeks of pregnancy, body growth slows as the baby puts on weight — mostly in the form of body fat — in preparation for birth.
The last four weeks of pregnancy are weeks 36 through 40. During this time, the baby gets ready to make her entrance into the world. Space is tight, so she doesn’t move around as often. Her lungs finally mature, and in most cases, she moves into a head-down position to make the birth process easier. She is also gaining body fat. A baby born during this time has over a 95 percent chance of surviving outside of the womb.
Babies born weighing less than 5 lbs. are considered premature. In most cases, this means they were born before the 36th week of pregnancy. During weeks 36 to 40, the baby rapidly gains body fat at a rate of at least 1/2 lb per week, according to the MayoClinic.com website. By the time a baby is full term, he usually weighs between 7 and 9 lbs.
Before the last month of pregnancy, your baby’s skin has very little fat underneath it. Instead, the skin is covered by a protective coating of vernix, which is a waxy substance, and hair. As your baby gains weight in the form of body fat, the hair and vernix dissipate. Some babies that are born before 40 weeks have a little bit of this protective covering and hair. In these cases, the hair quickly falls out and the vernix is absorbed into the skin.
Babies can gain too little weight in the womb. This condition is called oligohydramnios and is caused by a lack of amniotic fluid. Around 8 percent of women suffer from oligohydramnios during the third trimester of pregnancy, according to BabyCenter. In these women, their fluid levels are monitored carefully. In some cases, the baby is delivered early if the levels drop dangerously low.
It is a common misconception that a baby can gain too much weight during the last four weeks of pregnancy and be “too big” to be born. While it’s true that babies can be quite big, most women are still able to give birth, although these women might need extra help, such as an episiotomy. In rare cases, a cesarean might be needed for a very large baby. Babies that are over 9 lbs, 15 oz are considered to be “macrosomic,” or larger than average.