If you are pregnant with twins, the course of your pregnancy will be a bit different than if you were only carrying a single baby. For one thing, you will have to eat a bit more and gain more weight to ensure your babies emerge from the womb healthy. The rate of growth for the twins will be a bit different as well, since they are sharing a space designed for a single fetus.

Formation

Twins can form in one of two ways. Your ovaries may release two eggs at the same time. If this happens, fraternal twins form. Each egg is then fertilized by a separate sperm and implants on the wall of the uterus. As the fertilized eggs grow, each will have its own placenta and amniotic sac. In the case of identical twins, only one egg is fertilized. The egg then splits in half. Identical twins are much less common than fraternal twins. They share a placenta in the uterus, but will each have a separate amniotic sac.

Implantation

As with a single fetus, twins will implant on the wall of the uterus by the third week of pregnancy. At this point, each twin is a zygote made up of several hundred cells, according to BabyCenter.

Embryo Stage

Around the seventh or eighth week of pregnancy, each twin should have grown into an embryo. Their brains, joints and facial features will begin to form, as will their genitalia. If you are carrying fraternal twins, it’s possible you are carrying both a boy and a girl. Identical twins are always the same sex. The twins are still very tiny at this stage, usually no more than 1.25 inches by week 10.

Early Fetal Stage

At week 11, the twins enter the fetal stage. Each should be between 2 and 3 inches long by week 14. By the end of the second trimester, your twins will have grown to about 11 inches long and may weigh around 2 lbs. Their lungs will have begun to mature around week 24.

Third Trimester

While twins usually grow at the same pace as single babies in the first two trimesters of pregnancy, they start to slow their growth in the final trimester before a single baby would. Many twins actually develop more quickly than singles in the first two trimesters, according to Dr. Marjorie Greenfield of the Dr. Spock website. The fastest early development makes them more likely to survive a preterm labor, which happens often with twins. Usually, by the end of the pregnancy, each twin weighs about 6 lbs. and measures between 18 and 21 inches long.