Small-Baby Biology

Abstract

Second of two articles The normal course of fetal development can be thwarted by a multitude of factors originating in the mother or the fetus itself. When these adverse conditions result in retarded fetal growth, the infant is often at higher than normal risk for problems at birth and later in life. medical researchers are eager to develop methods to detect intrauterine growth retardation and to treat the fetuses, therapeutic measures will be difficult to develop and evaluate until the investigators better understand the various factors involved in producing small-forgestational-age babies and the reasons behind their developmental difficulties. Maternal factors appear to be most often responsible for the depressed rate of fetal growth, David R. Cox of tte University of California at San Francisco told a recent meeting at the National Institute for Child Health and Human Development in Bethesda, Md. He speculates, While maternally controlled restraint of fetal growth could be due to either limitation of space, limitation of essential nutrients, or restricted transfer of nutrients from the mother to the fetus, restriction of nutrient transfer is probably the most important factor in humans. The cellular mechanisms by which maternal genetic factors limit transfer of nutrients from the mother to the fetus are essentially unknown. Cox suggests maternal genes may control the development of the blood supply to the placenta and the fetus's supply of hormones and growth factors. Small fetuses tend to have small placentas, so blame for growth retardation is often fixed on the placenta. But it is not certain what is cause and what is effect. am not impressed with the prognostication from the size of the placenta to the size of the fetus ... nor do I believe that 'placental insufficiency,' as it is used in common parlance, exists at all, says Kurt Benirschke of the University of California School of Medicine at San Diego. Some placental characteristics that have been associated with intrauterine growth retardation, according to Benirschke, are absence of one umbilical artery, disease-related changes in the placental vascular bed in hypertensive toxemia, some inflammatory conditions, and a condition in which the placenta is small, thick and its characteristics suggest abnormal implantation. In addition, Benirschke has detected proteins characteristic of herpes viruses in the placenta of newborns with a variety of problems, including intrauterine growth retardation. One possible cause of intrauterine growth retardation is that during pregnancy a woman's blood vessels are only incompletely transformed into arteries capable of supplying blood to the uterus and placenta, according to F. Van Der Veen and H. Fox of the University of Manchester in England. Their microscopic studies suggest that placentas of small-forgestational age infants may receive insufficient oxygen. They therefore suspect that the primary cause of fetal growth retardation is generally not placental damage but rather restricted blood flow from the mother. Another likely candidate as a cause of intrauterine growth retardation is maternal nutrition. Historical data support a direct relationship between diet and fetal growth. During the Dutch famine of 19441945, the overall drop in birthweight in the previously well-nourished Dutch population was about 200 grams. Infants born during the siege of Leningrad, 1941-1944, showed an even greater reduction in birthweight, about 400 grams. Yet, in the United States today most mothers of small-for-gestational-age babies do not appear to be grossly malnourished, says Jack Metcoff of the University of Oklahoma in Norman. If fetal growth is modulated by maternal nutrition, then some sort of subtle malnourishment, for example a nutrient imbalance, must be present, Metcoff says. To find the underlying causes of fetal growth retardation, and to develop treatments for it, many researchers have turned to animal experiments. There, scientists can apply treatments that may improve the outcome of a pregnancy and indicate promising approaches for human obstetrics. In pigs there is a natural model for intrauterine growth retardation the fetuses at the far end of the uterus appear to receive less nourishment and are born the runts, substantially smaller than their littermates, although brain development lags less than does body size.