The Stages of a Twin Pregnancy

Bearing twins is a little bit different than being pregnant with a single child. The child development in the womb is similar but slightly altered. It also depends on whether the twins are fraternal or identical. The development of fraternal twins starts with two separate eggs; identical twins develop when one egg separates and becomes two.

Pregnancy is divided into trimesters, with each lasting about three months. The first two trimesters are the most similar to single pregnancies, however, the weights and lengths of twins are a little smaller than those in a single birth. By the end of the first trimester the organs, extremities and brains have begun to develop and the festuses each will weigh about a half-ounce. At five months they each weigh about 1 pound and are about 8 or 9 inches long. By the end of the second trimester, the twins' bones have formed and fat has started to accumulate. The organs continue to develop, and hearing becomes possible. The fetuses weigh about 2 pounds and are 9 inches long at the end of the second trimester.

Starting in the third trimester twin pregnancies start to vary more. Twins grow slightly faster when they reach the seventh month. Then in a complete reversal, they slow their growth before a single child would. Because they slow earlier, they are typically smaller. Healthy twins on average, in a healthy mother with no complications, will be delivered around 37 weeks, This is three weeks before a single birth, and is considered full term. It is also another reason that twins typically weigh less at birth.