Your baby grows and changes from a small group of cells to a fully developed baby ready to begin life. The designation between "embryo" and "fetus" is somewhat arbitrary. During the first 8 weeks of development (10 weeks of gestation), the developing baby is called an embryo. From 8 weeks of development until delivery, it is called a fetus. The great changes your baby goes through to become a fully developed baby are easier to follow if we look at them in each trimester.
Trimesters
The length of your pregnancy is divided into three trimesters, each about 13 weeks long.
First trimester development.
The first trimester represents the greatest change for any developing fetus. In the first 13 weeks of development, your baby grows from a collection of cells the size of the head of a pin to a fetus the size of a softball. Organs begin developing, and your baby begins to look more like a baby.
Very few, if any, structures in the fetus are formed after the 12th week of pregnancy. This means your baby forms all of its major organ systems by the end of the first trimester. These structures continue to grow and to develop until your baby is born.
Second trimester development.
At the beginning of the second trimester (14th week), your baby weighs less than 1 ounce (28g) and is only about 4 inches (10cm) long.
Third trimester development.
Your baby weighs about 1-1/2 pounds (0.7kg) at the beginning of the third trimester (27th week), and its crown-to-rump length is under 9 inches (22cm). {Crown-to-rump length is the measurement from the top of the baby's head [crown] to the buttocks of the baby [rump].) When it is delivered, your baby will weigh close to 7-1/2 pounds (3.4kg) and be about 21 inches (53cm) long.
Baby's Size and Weight
Birthweight varies greatly from baby to baby. However, the average weight of a baby at term is 7 to 7-1/2 pounds (3.3 to 3.4kg).
Ultrasound is the test of choice to estimate fetal weight. A formula has been established to help estimate fetal weight using this technology. Several measurements are taken, including the diameter of the baby's head, circumference of the baby's abdomen and length of the femur (thighbone) of the baby's leg. Occasionally other fetal measurements are taken. A drawback of using ultrasound for estimating fetal weight is that estimates may vary as much as half a pound (225g) in either direction. However, the accuracy of predicting fetal weight with ultrasound continues to improve.
The size of the fetus's head surprises many of my patients. When you are 13 weeks pregnant, your baby's head takes up about half the crown-to-rump length (measurement from top of the head to the baby's buttocks). In 2 months, when you are 21 weeks pregnant, the head will be about one-third of the fetal body. At birth, your baby's head will be one-fourth the size of its body.
