IOWA CITY, January 14, 2011 ( - A project by Dr. Edward Bell of the University of Iowa, in Iowa City, to create a registry of surviving infants with birth weights of less than 400 grams has found that, while still rare, the chances of survival of these tiny children is rising.

The “Tiniest Babies Registry” has compiled data on 110 babies born between 1936 and 2010 weighing between 260 and 397 grams at birth and having gestational ages from 21 to 34 weeks.

Dr. Bell told Reuters Health that he was motivated to create the registry in 2000 after a baby girl, now patient #11 in the Tiniest Babies Registry, was born at his university hospital weighing just 359 grams.

“When our patient survived, I began to look around to see what other tiny survivors had been reported,” Bell said.

Dr. Bell and fellow researcher Diane Zumbach found that the number of “micro premies” who survive each year has increased since the early 1990s, and noted that gestational age was more important for the babies’ survival than their size.

“By far, the vast majority of infants born alive weighing less than 400 grams are too early in pregnancy to survive,” Dr. Bell said in the Reuters report, indicating that the children in the survivors registry were unusually small for their gestational ages but more fully formed than an average 400-gram baby.

“A normally-grown 400-gram baby would be approximately 19 weeks along in pregnancy, which is 3 to 4 weeks before reaching a level of development that allows even a chance of survival outside the womb,” Bell said.

The researchers also noted that female babies had a much better chance of survival than males, conjecturing that female hormones may play a part in the earlier maturation of internal organs.

“Eighty-three (75%) of the patients are female. The 10 smallest infants are female, and the registry contains only 1 boy who was born weighing less than 300 g,” Bell wrote in a report published in the journal Pediatrics.

Dr. Bell cautioned, however, that information on long-term health of the children is limited and that many of them have ongoing health and learning problems.

“Since the birth of the first survivor below 400 grams in 1936, there have been something like 10 billion babies born in the world who survived to go home with their parents, and we know of only a few more than 100 of these who weighed less than 400 grams,” Dr. Bell told Reuters.

“Undoubtedly, there are more that have not yet made it to the Registry,” he added. “In fact, I found another baby shortly after the report was published. Patient #111 is not included in the paper, but he is the smallest boy to survive at 274 grams.”

An abstract of the report by Dr. Bell, titled “The tiniest babies: a registry of survivors with birth weight less than 400 grams” is available here.

The “Tiniest Babies Registry,” which has a list of the babies in order of date of birth and birth weight, is available here.