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| |    Twins: Fetal Lengths and Weights at Different Gestations by Lisa Rodriguez, R.N. and Marjorie Greenfield, M.D. reviewed by Laura Jana, M.D., F.A.A.P. Most people are intrigued with the idea that humans are capable of carrying more than one baby through a pregnancy. Not surprisingly, this interest is even more pronounced in the parents-to-be of twins, who likely are curious about how their pair's growth and development differs from that of singletons. During the first two trimesters of the pregnancy, twins typically develop at the same rate as single babies. In the third trimester, however, the amount of weight gained by the two infants together is about equal to the amount of weight gained by the one infant in a singleton pregnancy. In other words, while a singleton's weight climbs until 40 weeks' gestation before slowing down, multiples have deceleration of growth much earlier--seen typically in twins at 30 to 32 weeks. Consequently, twins and other multiples tend to be smaller than singletons at the same gestational age. Early in the third trimester, twins actually may have an advantage when it comes to growth, because they generally develop slightly faster than singletons do until about 30 weeks. For example, a set of twins delivered at 30 weeks will probably have better lung development and may have fewer problems due to a premature birth than a singleton born at the same gestation. The lengths below given for the first several months of pregnancy are calculated in crown-to-rump measurements (that is, from the top of the fetus's head to his buttocks). Crown-rump measurements are used because it's difficult to get a true length measurement during these early months due to the curled position of the fetus. 1 month (6 weeks) Each of your babies is about 1/10 of an inch long. This is about the size of a grain of rice. 2 months (10 weeks) Your babies are about 1¼ inches long, crown to rump, and weigh about 1/3 ounce each--about the size of a golf ball. 3 months (14 weeks) Your twins are probably about 2½ to 3½ inches, crown to rump, and weigh about 1 ounce each. Both babies are now a little smaller than the size of your fist. 4 months (18 weeks) Your twins are likely to measure about 4½ to 5½ inches from crown to rump. Each weighs about 5½ ounces. 5 months (22 weeks) Both babies are about 7 to 10 inches long, head to toe, and weigh about 1 pound each--the size of a large banana. 6 months (26 weeks) Your bundles of joy are about 11 to 14 inches long, weighing 1½ to 2½ pounds each. 7 months (30 weeks) At this stage, each baby will measure around 16 or 17 inches and weigh about 3 to 3½ pounds. 8 months (34 weeks) By this time, your twins are probably somewhere between 17 and 19 inches long, weighing between 4 and 5 pounds each. You are now carrying more baby than most singleton moms carry at 40 weeks. No wonder you're tired! 9 months (37 to 40 weeks) On average, at term--37 weeks for twins--each baby is 18 to 21 inches long and weighs about 6 pounds.
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