|Causes and Consequences of Intrauterine Growth Retardation, Proceedings of an IDECG workshop, November 1996, Baton Rouge, USA, Supplement of the European Journal of Clinical Nutrition (International Dietary Energy Consultative Group - IDECG, 1996, 100 pages)|
|Biological mechanisms of environmentally induced causes of IUGR|
There is an association between parity and birth weight. Primigravidae are more likely to give birth to small for gestational age babies than multiparous women. In the British Births Survey of 1970 (Chamberlain et al, 1975) firstborn babies were lighter than babies born from multiparous mothers, especially babies born after less than 37 weeks of gestation. Moreover, Billewics and Thomson have shown an increase in birth weight over the woman's whole reproductive life (Billewicz and Thomson, 1973). Carr-Hill and Prithchard (1985) however, reached the conclusion that increases in birth weight from one child to the next was more closely related to maternal weight before successive pregnancies than to parity.
In a pregnancy with a normally grown fetus the maternal plasma volume increases by about 50% with possibly an additional 5% in multigravidae. This increase and the birth weight are directly related (Hytten and Paintin, 1963). In fetal growth retardation the increase in plasma volume may be only half of that in a normal pregnancy. The increase in plasma volume is least in those who have a history of recurrent abortion, or IUGR. If plasma volume expansion is less than normal but the red cell volume increases normally, then the plasma viscosity rises. This increase of plasma viscosity is not linear but exponential at high haematocrits. The consequence of this is a reduced capillary flow and increased tendency to thrombosis. A maternal hemoglobin concentration over 12g % can result in placental infarcts and IUGR (Naeye, 1977).
About a third of multiple pregnancies finish preterm. In addition these babies also show IUGR compared with singleton babies. Monozygotic twins tend to be more growth retarded than dizygotic twins (Bulmer, 1970). A similar association has been reported in mono-amniotic twins (Wharton et al, 1968).
With increasing height above sea level, the atmospheric pressure is reduced and therefore the partial pressure of oxygen decreases. Sabrevilla et al (1968) showed a marked reduction in birth weight in babies born at extremely high altitudes. McCullough et al (1977) found a greater proportion of babies with IUGR among those born at a high altitude in the Rocky Mountains than among babies born in Denver, and Gibson et al (1973), showed an inverse relationship between mean birth weight and hemoglobin levels in pregnancy. High hemoglobin levels were associated with low increases in plasma volume, and this may be the real reason for the negative association between altitude and fetal growth.
Finally, one of the strongest indicators of a multiparous woman's performance is her previous obstetrical performance. It is conceivable that the increased risk of producing recurrent IUGR exists because of characteristics of the mother rather than an assigned medical reason.