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Objectif Nutrition N°55 (January 2000)
BODY WEIGHT AND PREGNANCY
Dr. Martine PELLAË

Excess body weight and obesity in pregnant women, but also familial or acquired underweight, can be the cause of pregnancy-related or neonatal complications. Improving the mother's nutritional status from the time before conception can limit the risks which compromise the newborn's future.

A national survey conducted in 1998 on 14,088 women shows that general health status at birth is constantly improving. And yet prematurity and newborn hypotrophy continue to account for some 40% of hospital admissions.

There is one determining factor among the many involved in the harmonious course of pregnancy and delivery: a good balance between dietary intake and the recommended allowances. Adequate weight gain reflects a well-adapted calorie intake, at least on a quantitative level. While it is not necessary to greatly alter the diet of a normal-weight pregnant woman, conversely, it is necessary to restore a balance in the dietary intake of at risk women. Indeed, excess weight and obesity, as well as familial or acquired underweight, can be the cause of pregnancy-related or neonatal complications.

I - ENERGY ALLOWANCES IN PREGNANCY

During pregnancy, energy allowances should ensure the maintenance of newly-formed maternal and fetal tissues, the increase in basal metabolism associated with an increase in tissue mass, the rise in the mother's body stores (lipids in the mother's adipose tissue) and the requirements for the fetus and the associated organ systems (uterus, placenta, amniotic fluid).

During the anabolic phase of development (first two trimesters of pregnancy), calorie allowances of fetal growth are low and are due especially to the increase in size and storage of energy and nitrogen in certain tissues (breasts, uterus and associated organs). At a term of 6 months, the fetus barely weighs more than one kilogram and thus it is possible for the mother to build up adipose tissue stores up until month 6 of pregnancy in preparation for the third trimester of pregnancy.

During the catabolic phase of development (third trimester), the fetus uses the mother's body stores to meet energy requirements necessary for growth. Starting at week 25, the increase in the weight of the mother's associated organ system and that of the fetus is exponential. The fetus, which up until then gained 5 to 6 g a day now gains 20 to 25 g a day on average (see Table 1).

Table 1 : DISTRIBUTION OF WEIGHT GAIN DURING PREGNANCY

Weight gain Week 10 Week 20 Week 30 Week 40
Fetus (g) 15 320 1.500 3.500
Placenta and amniotic fluid (g) 40 400 1.050 1.250
Uterus and breasts (g) 150 750 1.100 1.300
Blood (g) 100 500 1.100- 1.300 1.100 - 1200
Interstitial fluid (g) 100 200-400 400-800 1.000 - 1.200
Adipose tissue reserves (g) 200-300 800-1.600 2.000 - 3.500 2.000 - 4.000
TOTAL 600-700 3.000 - 4.000 7.000 - 9.000 12.000 - 12.500

Out of 12 kg gained during pregnancy, only 2 to 4 kg consist of adipose tissue.

In "Importance de la nutrition prénatale", pp 34-48 in "Nutrition et renutrition en pratique pédiatrique", C. POLONOVSKI et al., Expansion Scientifique Française, 1992.

II - METABOLIC ADAPTATION IN PREGNANCY

Some women are overweight at the time of conception, or gain 20 kg during pregnancy; conversely, women of low-body weight with little body stores can lose adipose tissues during gestation. How can such pregnant women with such a different nutritional status deal with the dynamic anabolic condition corresponding to pregnancy? There are three potential mechanisms of adaptation: an increase in the mass of fatty tissue (+ 3 to 5 kg), a reduction in physical activity, or modulation of basal metabolism.

The difference between a well-nourished woman and an undernourished woman lies in the body fat stores built up until week 20 of pregnancy: 3 to 4 kg are adequate to meet maintenance requirements. During the anabolic phase of pregnancy, the massive increase in progesterone secretion produces a decrease in energy consumption at rest by about 10%: low-energy requirements are associated with energy sparing. Thus, during the first two trimesters, if the mother's diet is normal, varied, balanced and not restrictive, it is not necessary to advise her to increase her dietary intake in order to ensure harmonious development of the fetus and the maintenance of a satisfactory, healthy condition in the mother-to-be. Thus reality contrasts with the traditional adage "A pregnant woman should eat for two." It is only in cases of malnutrition that energy allowances should be increased since there is a relationship between the mother's body weight before pregnancy, her weight gain during pregnancy, and the weight of the baby at birth.

During the third trimester, fetal energy requirements, while increasing, remain low (100 to 120 kcal/day during month 9). Thus it appears adequate for the pregnant woman to moderately increase her daily calorie intake (by about 150 kcal/day) during the last trimester of pregnancy. It is then sufficient to recommend that she eat an additional snack during the day.

III - LIMITS OF ADAPTATION

Many epidemiological studies have shown that an energy intake that is too low produces a higher risk of fetal hypotrophy (birth weight less than 2.5 kg). And yet, perinatal mortality is that much higher when the baby has a low-birth weight. This situation, frequently occurring in developing countries, is not rare in France in women from underprivileged backgrounds or in those who have followed very restrictive diets or diets which are inappropriate for the nutritional requirements of pregnancy.

In the longer term, it is possible that fetal adaptation to nutritional deficiencies induces metabolic sequelae: studies suggest the fetal origin of certain diseases occurring in adulthood.
Indeed, babies born to undernourished mothers can develop a metabolism of energy sparing, "saving," predisposing them to obesity disorders because this metabolism cannot adapt to subsequent excessive nutritional intake.

The inability of a malnourished woman to build up adequate body fat stores partially accounts for the geographic disparity observed in terms of low-birth weight babies: 6% in Europe, a figure which is more than double in Africa (14%) and five times more in southwest Asia (31%). Infectious and parasitic diseases, early pregnancies or closely spaced pregnancies are as much etiologic factors but diet plays a dominant part as well.

IV - PREGNANCY AND IDEAL WEIGHT GAIN

During the first nine weeks of pregnancy, weight gain is low because the fetus weighs only 5 grams at the end of this period. Generally, weight gain is slow up until week 18, and then accelerates up until week 28 to subsequently slow down before delivery.
Independently of total weight gain, inadequate weight gain up until the end of the second trimester multiplies the risk of intra-uterine growth retardation by a factor of two and that of low-birth weight fetus by a factor of three. In the last trimester, a low-weight gain doubles the risk of prematurity.

If a normal-weight woman, with a balanced diet, does not to modify her diet during pregnancy, the same is not true for overweight or underweight women. In case of women who are overweight or obese, theoretically it would be desirable to have a more balanced diet and to lose weight prior to conception. If this has not been possible, the goal should not be to attempt to make a pregnant woman lose weight, since it then would be difficult to meet the requirements for trace elements: thus overly restrictive diets should be avoided (less than 1700 kcal/day). On the other hand, it is logical to limit weight gain, since weight gain during pregnancy increases the risk of medical and obstetrical complications which may persist after delivery in women with a high body mass index (BMI).
Conversely, in women with a nutritional risk (see Table 1) and in very thin women, it weight gain must be promoted during pregnancy in order to prevent a very low-birth weight infant (see Table 2).



INSERT 1 : WOMEN AT RISK FOR INADEQUATE WEIGHT GAIN
  • Teenagers
  • Women with nutritional risks :
    • underprivileged background
    • vegans
  • very thin women
    • multiple pregnancies
    • women on self-imposed diets
    • women with substance abuse problems (tobacco - alcohol - drugs)
  • Women with a previous history of :
    • pre-term delivery
    • low-birth weight babies
    • intra-uterine growth retardation
Table 2 : RECOMMENDED WEIGHT GAIN DURING PREGNANCY BASED ON THE MOTHER'S BODY SIZE AT TIME OF CONCEPTION

BMI Weight to be gained (in kg over 9 months)
< 19,8 12,5 à 18
19,8 - 26 11,5 à 16
26 - 29 7 à 11,5
> 29 6 à 10

In Abrams B. et al., Am. J. Clin. Nutr. 2000, 71 : 1233S - 1241S.

V - RESTRICTIVE DIETS AND THE FUTURE OF THE NEWBORN

A restrictive diet is the cause of risks, particularly in women who, during pregnancy, continue with their obsessive quest for slimness and in women with normal weight, "victims of the fashionable suit syndrome" i.e., the suit that they wish to be able to wear again after the birth of their baby.
The almost obsessive compliance with a diet inappropriate to meet the mother and fetus' requirements can cause fetal malnutrition resulting in a risk of growth retardation, and thus in a low-birth weight baby. Studies have shown that in addition to the immediate risks in the neonatal period, low-birth weight babies subsequently in adulthood are more prone to develop hypertension, abnormal carbohydrate metabolism and coronary heart disease.

INSERT 2 : RISKS ASSOCIATED WITH AN OVERLY-RESTRICTIVE DIET

During trimesters 1 and 2 Risk of fetal hypertrophy and intra-uterine growth retardation.
Trimester 3 Increased risk of fetal prematurity.


VI - CONCLUSION

Despite the multiple potential mechanisms for physiological adaptation, an adequate energy intake is essential to optimize a baby's birth weight, which is an essential criterion in determining neonatal morbidity and mortality.
An individual medical consultation makes it possible to take into account the nutritional status and BMI of the pregnant woman at the time of conception, her previous medical history, her lifestyle and her eating habits before and during pregnancy.

Ideally, the mother's nutritional status should be improved before conception.


BIBLIOGRAPHIE

Bringer J., Galtier-Dereure F., Boegner C., Boulot P. Influence du poids sur la grossesse. Reproduction humaine et hormones.
1998 ; 11 : 533 - 539.

Papiernik E., Potier de Courcy G.L'alimentation pendant la grossesse et l'allaitement. La nutrition humaine.
INSERM. Nathan, Paris. 1996 ; 105 - 110.

Putet G. Besoins nutritionnels de la femme enceinte.
Arch. Pédiatr. 1997 (suppl 2) : 131 -134.

Valat A.S. Conséquences de la maigreur et de l'obésité sur la grossesse et l'accouchement.
Rev. Fr. Gynécol. Obstét. 1999 ; 94 : 384 - 387.

L'alimentation de la femme enceinte. Recommandations et Conseils pratiques.
Brochure CERIN.



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