A woman gave birth to octuplets in California Monday after 30 weeks of pregnancy. The six boys and two girls ranged in weight from 1 pound, 8 ounces, to 3 pounds, 4 ounces. How many babies can fit inside a pregnant woman?
There's no scientific limit, but the largest reported number of fetuses in one womb was 15. In 1971, Dr. Gennaro Montanino of Rome claimed to have removed 15 fetuses from the womb of a 35-year-old woman. The largest number of babies ever delivered was in Australia in 1971, when a 29-year-old women gave birth to nonuplets. They all died. (Decaplets were supposedly born in Brazil in 1946, but that report is unconfirmed.) The largest set of children to be born together and survive—not including this week's octuplets, who are all still alive—is seven. A set of septuplets born in Iowa in 1997 became the first to survive infancy. At least two more sets of septuplets have since been born and survived.
How do so many babies fit in one woman? The limit isn't so much the number of babies as their volume and weight. In general, once the total weight of the babies inside reaches about 12 pounds, the uterus goes into labor. The greater the number of fetuses, the earlier the labor will occur. (The rough formula for due dates is to start at 40 weeks of gestation for one baby and subtract three weeks for each additional child: 37 for twins, 34 for triplets, etc. It's remarkable, then, that eight fetuses were able to gestate for 30 weeks, as in the California case.) Labor can also occur if a sac ruptures and introduces bacteria into the uterus or if one of the fetuses isn't growing correctly and signals the uterus to induce labor. (Scientists still don't know exactly what causes labor to start.) But for safety reasons, multiple births are often carried out before labor naturally occurs, and almost always by cesarean section. If the uterus gets too big and the fetuses too heavy, it can be difficult for the woman to breathe. The amount of placental tissue can also cause the mother to develop pre-eclampsia, or pregnancy-related hypertension.
Don't the babies get tangled up? Usually not. * Normally, each baby is enclosed in its own amniotic sac, which keeps the various umbilical cords and limbs from getting intertwined. Picture a bucketful of water balloons. In each sac, there's plenty of fluid—about two-thirds the volume of the fetus—for the baby to move around in. After the sacs break, the babies can get tangled up during birth—so-called interlocking twins—which is why doctors usually opt for cesarean section. It is possible to have monoamniotic twins, or fetuses that share the same sac, with dangerous results: Cords can get compressed or entangled and cut off the babys' oxygen and food supply.
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Explainer thanks Joshua A. Copel of Yale University Medical School and Thomas Moore of University of California, San Diego.
Correction, Jan. 30, 2009: This article originally stated that fetuses in multiple births always have their own amniotic sacs. ( Return to the corrected sentence.)