The ball of cells embedded in the uterus is already laying down the foundations for its future life as an embryo. At two layers thick, the germ cells form a flat disk that divides the fluid-filled inner part of the ball of cells into two chambers. The smaller of these fluid-filled chambers will become the amniotic sac. The larger chamber, lying closest to the future placenta, will become the yolk sac that supports the early embryo. The umbilical cord will eventually develop close to the smaller chamber. The inner germ cells have been developing at a slower rate than the rapidly expanding outer cell layers.
At first the umbilical cord is a simple stalk, containing no blood vessels but simply anchoring the embryo to the future placenta (see You are 4 Weeks and 6 Days), which will eventually become your unborn baby's lifeline.
Newborns are getting heavier.
This is mostly due to improved diet and living standards. However, obesity in the mother is another factor-if the mother is overweight, there is an increased risk of diabetes, which can increase the baby's weight.
These days, it is almost impossible to pass a newspaper stand without seeing the latest celebrity who has not only fit right back into her clothes after having her baby, but who actually weighs less than she did before pregnancy. However, this is concerning for health professionals, since a dramatic weight loss after the birth is not good for mother or baby.
The recommended weight gain during pregnancy is 25-35 lb, if you have a Body Mass Index (BMI) within the normal range. Your baby and her support system will make up a good proportion of this (see How much weight will you gain?), as will the increased pregnancy fluids, fats, and an enlarged uterus. Much of this extra weight will be lost as soon as your baby is born. Also, after the birth, some of this extra weight provides nutrients for breast-feeding, which uses up to 500 calories a day.
The most sensible approach to controlling your weight during pregnancy is to eat a healthy diet and get gentle exercise to ensure that weight gain is not too dramatic. You should be aiming to eat around 2,100-2,500 calories a day, increasing this by 200 calories in the last trimester of pregnancy-the equivalent of a banana and a glass of milk.