The Stages of a Twin Pregnancy

A twin pregnancy is different from a single pregnancy in several important ways. The development of twins generally mimics the development of a single baby, but sharing a uterus makes a twin pregnancy distinctive.

Twins' early growth in the womb differs between fraternal twins and identical twins. Fraternal twins are conceived when two eggs are released by the ovary and each is fertilized by a different sperm. They share the same space in the uterus. Identical twins are conceived when a single sperm fertilizes a single egg and the resulting zygote divides into two separate parts, each with the same genetic makeup.

A twin pregnancy is considered full-term at 37 weeks, shorter than a single birth. Twin babies will have fully mature lungs at birth, but will likely be smaller and a lower weight than a single baby.

Early Stages of a Twin Pregnancy
Most parents of twins find out they're having multiples from a routine ultrasound performed between the 16th and 20th week.

During the first two trimesters of pregnancy, twins develop at about the same rate as single babies. At 6 weeks, each of the twins is about 1/10 of an inch long, about the size of a grain of rice. At 5 months, each twin is 7-10 inches long, about the size of a large banana, and weighs about 1 pound each.
The Last Trimester of a Twin Pregnancy
In the third trimester of a twin pregnancy, growth patterns become distinctive and differ from single pregnancies. Early in the third trimester twins begin to develop slightly faster than single babies, until about 30 weeks. 

Then twins' growth and weight gain decelerates earlier than in a single pregnancy.  Consequently, twins are normally smaller and weigh less at birth than single babies.

At 7 months, twins weigh roughly 3 pounds each and at 8 months, twins can weigh between 4 and 5 pounds each.
The End of a Twin Pregnancy: Milestones of Twin Births
How long the mother can carry twins determines their chances of survival and their health as newborns.

Twenty-four weeks is known as the threshold of viability. Babies born at this stage will probably spend at least three months in the neonatal intensive care unit. A third of them will survive without long-term health problems.

At 28 weeks, viability increases drastically. Almost all babies (99 percent) who make it to this stage survive, however some suffer from complications.

Babies born at 32-34 weeks have an excellent chance of survival without any long-term complications. If their lungs are not fully developed, the babies may have to spend a short time in the NICU or may need oxygen or a feeding tube.

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