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The mother's nutritional status before conception (reflected by maternal BMI) and her weight gain during pregnancy are important determinants of fetal growth in developing countries and, to a somewhat lesser extent, in developed countries. In initially thinner women, the effect of weight gain on fetal growth is greater than in fatter ones. In very thin women, even a high weight gain during pregnancy may not be able to fully compensate (even with the interaction effect) the higher risk of having an IUGR baby. Being well-nourished before conception has a buffering effect and, in women with a prepregnancy BMI > 27, weight gain during pregnancy will no longer have much of an effect on fetal growth.
The Subcommittee on Nutritional Status and Weight Gain During Pregnancy of the US Institute of Medicine made recommendations in 1990, recognizing the relationship between prepregnancy BMI and desirable weight gain, but looking at outcomes in babies, not mothers. It recommended weight gains that, throughout the range of prepregnancy BMIs, were considerably higher than recommendations in other countries, including Europe. Several authors have drawn attention to the fact that the residual weight the mother retains after giving birth is dependent on the weight she gained during pregnancy. If recommended weight gains during pregnancy are relatively high, this results in a tendency to increasing BMIs after each pregnancy and can become a public health concern. The effects of recommending high weight gains during pregnancy on the birthweight distribution is less clear; a shift to the right would mean fewer IUGR babies and that would be an advantage; but it would also mean more macrosomic babies and possibly more complications at birth and a greater percentage of mothers needing Cesarean sections, which would be a disadvantage. An intriguing observation is that black women in the US tend to have babies with a lower mean birth weight, even though their body weight tends to be higher than that of white women. Making recommendations on weight gains during pregnancy is not an aim of this workshop, but several discussants are of the opinion that the next committee that will be charged with making such recommendations in the US may have to correct current estimates downwards.
A more general question is whether making recommendations has any effect on observed weight gains. What little literature exists on this subject suggests that recommendations given by health providers to pregnant women can and often do result in a change in knowledge but rarely produce changes in behavior and practice. Three randomized trials on the effect of nutritional advice given to pregnant women are not very strong methodologically. They show only a very modest effect on maternal weight gain and practically no effect on birthweight. On the other hand, it looks as if a more relaxed, less restrictive general attitude towards weight gain has led to a secular increase in birthweight, primarily among term infants. Unfortunately, this would also affect women with a high prepregnancy BMI.
A widely held belief is that since fetal growth is greatest in the third trimester, interventions are also likely to have their greatest effect during the third trimester. The Dutch women whose third trimester of pregnancy coincided with the famine towards the end of WW2 were the ones who gave birth to the lightest babies, and in The Gambia the prevalence of LBW babies is highest in women whose third trimester of pregnancy coincides with the hungry season in summer. Dietary supplementation of Taiwanese women before and during the whole pregnancy did not have more of an effect on birthweight than did similar supplementation trials during the last trimester only. However, the evidence bearing on earlier nutritional effects is not completely uniform. A study among adolescents showed that greater weight gain early in pregnancy had more of an effect than larger weight gains later. A study of rural Guatemalan women showed an interaction between gastrointestinal parasitosis and maternal height. In women of average height and weight, mebendazol therapy had little effect on birthweight, but in short women it did. In this study too, therapy had a greater effect during the first half of pregnancy than during the second half.
The assumption is frequently made that early growth restriction will result in small but proportionate babies, whereas growth restriction during the latter part of pregnancy will result in wasted babies of approximately normal length. Kramer argued that this generalization had its limits because of a strong association between wasting and severity of growth restriction. The theory explaining the high rate of growth-retarded babies born to teenagers by a competition for nutrients received support from a recent study in which knee height (as an indicator of bone growth) was monitored in pregnant teenage mothers. The birthweights of the babies were inversely related to maternal bone growth.
Genital and urinary tract infections
seem to result more in preterm births than in IUGR.