Prenatal influences on postnatal growth: Overview and pointers for needed research

1 Maternal and Child Health Program, and Department of Pediatrics, University of California, Berkeley and San Francisco, CA, USA;

² Department of Obstetrics and Gynecology, University of Münster, Germany

Correspondence to: F. Falkner.

1. Introduction and background

In many discussions of linear growth retardation there is a need to address arguably the most important phase of human growth - growth from conception to term. Only comparatively recently have studies focused on fetal growth and outcome after gleaning important data from intrauterine study - difficult though such studies are.

Questions that come to mind include: Can children be programmed in utero to be linearly growth retarded after birth? What are the relationships between intrauterine growth retardation (IUGR) and postnatal growth? Depending on its growth in utero, the newborn baby can be categorized as a healthy full-term (FT) infant within the normal birth weight range; a macrosomic infant above the normal birth weight range; or an infant of low birth weight (ILB). Regarding the latter, WHO estimates are of great interest: they show that approximately two-thirds of all infants of low birth weight born in the developed world are true pre-term (PT) infants and one-third are small-for-gestational-age (SGA). This relationship is reversed in the developing world, where about 75% of ILB are SGA. The much higher proportion of SGA infants in the Third World seems to be due primarily to malnutrition and infection and should therefore be preventable.

2. Fetal growth

During the embryonic period the rate of weight gain of the fetus is not very high. Its subsequent increase and decrease are mainly a reflection of cell multiplication. A period of linear growth peak velocity, as in adolescence, can be assumed, but the gestational age at which it occurs is still somewhat uncertain. Thence, linear growth deceleration occurs and continues in the postnatal period. Weight gain of the fetus follows the same general pattern as growth in length, except that peak weight velocity is thought to occur later.

There is a discontinuity between fetal and infant growth curves. Adaptation appears to occur. There is considerable evidence that, beginning at 34-36 weeks, fetal growth slows down, owing to space constraints within the uterus. Twins slow down demonstrably earlier when their combined weight is approaching that of a 36-week singleton fetus.

A hypothetical curve of weight gain velocities connecting a peak weight velocity point at, say, 32 weeks, with a weight gain velocity point 8 weeks post partum, clearly takes no account of the slowing in growth just described. Infants usually lose some weight immediately after birth which they then rapidly regain. On average, catch-up growth is greatest in those infants most delayed in utero. As a result, there is a significant negative correlation between birth weight and weight gain in the early postnatal months. The same phenomenon occurs for length; the smaller the infants, the more, on average, they grow in the postnatal period.

It might be expected that any maternal factor regulating fetal growth would act protectively to prevent serious depletion of maternal nutrient and mineral supplies by slowing down fetal growth. But near normal fetal growth even in severe maternal malnutrition would indicate that there are placental and fetal growth-controlling factors that operate normally under these circumstances. How is it, then, that especially the nutrition component of poor and adverse environmental conditions is associated with lowered human birth weight? This is largely due to factors causing a marked reduction in growth rate after 36 weeks, for the mean birth weight of infants born at 36 weeks in many parts of the world under different circumstances is quite similar.

A mother not achieving her own growth potential, due to malnutrition or other adverse environmental factors, is likely to have smaller fetuses and newly born infants than had she grown up in good circumstances. Clearly, two generations or more may be needed to reverse the effects of a poor environment on size at birth. Guatemalan mothers of short stature had babies of smaller size than did mothers of medium stature. A food supplement given to both samples of mothers during pregnancy increased the birth size of the small mothers' infants more than the size of infants of larger mothers. It did not eliminate the difference completely, but nearly so (Lechtig & Klein, 1980).

3. Intrauterine growth retardation

It is not intended to discuss causation of IUGR, but only to consider its possible relationships with postnatal growth.

Utero-placental and umbilical blood flow can be important factors, as can the transfer of glucose through the placenta or the production of fetal insulin. The roles of fetal pituitary growth hormone and fetal thyroid hormone need continuing study; anencephalic and athyroid fetuses, for example, seem not to exhibit fetal growth retardation (Vorherr, 1982). Placental lactogen and somatomedin and a somatostatin-like substance of the placenta and fetus may well influence fetal growth, and animal experiments certainly suggest the assistance of placental-fetal growth-promoting and - controlling factors. Their identification in the human would contribute much to our knowledge of fetal growth, health and adaptation.

Regarding the placenta, Lechtig et al. (1975; 1977) indicated that nutritional supplementation of the mother during pregnancy is associated with improved placental weight and higher levels of alkaline ribonuclease activity. Leaf et al. (1992), in a very recent study of essential fatty acids at birth, showed that placental function is important in the transfer of some fatty acids from mother to fetus, and that these fatty acid levels were correlated with fetal growth and maturation in the premature infants they studied. Clearly, the placenta has very complex metabolic and endocrine functions. IUGR associated with placental dysfunction tends to occur later in pregnancy and head growth appears spared, whereas early onset IUGR tends not to spare head growth (see later discussion on SGA infants).

It is well established that malnutrition and infection can cause IUGR. During the past decade it has been documented that prenatal intrauterine infection increases fetal IgM. Mekki et al. (1988) and Lechtig et al. (1974) sampled cord blood and showed that IgM levels were higher in newly born infants from very poor environmental backgrounds.

There is certainly a pressing need for detailed studies on the role of infection in IUGR, particularly in tropical and underdeveloped countries. Pardi et al. (1993) showed that techniques for sampling cord blood in utero (cordocentesis) offer the opportunity to assess a fetus's metabolic environment before parturition. In fetuses with IUGR, the detection of, for example, hypoxia, acidemia, low amino-acidemia, and endocrine abnormalities, creates a greater opportunity to study many aspects of not only IUGR but also fetal growth. Kempley, Gamso & Nicolaides (1993), using Doppler ultrasound, measured left renal artery blood flow in the first postnatal week of very low birth weight SGA infants. Compared with weight-and gestation-matched controls, SGA infants had significantly lower blood flow velocity. Thus, abnormalities of blood flow velocity appear to persist after delivery in those SGA infants.

Doyle et al. (1990) found positive correlations between maternal nutrient intake, assessed during one week towards the end of the first trimester of pregnancy on the one hand, and weight, length and head circumference of the newborns on the other, especially in infants with birth weights below 2500 g. Vitamin/mineral supplementation of mothers during the last two trimesters of pregnancy, however, had no significant effects on birth dimensions.

4. Small-for-gestational-age infants

Prenatal intervention that would prevent low birth weight infants who are SGA would be a major contribution towards perinatal health improvement. We need, then, to search not only for causes of SGA (of which there are clearly many), but also for methods for predicting their linear growth. On the one hand, there are SGA infants growing very poorly after birth; on the other there are SGA infants who grow very well and achieve their genetic potential. It is commonly, and quite wrongly, taught that all SGA infants with growth retardation tend to catch-up in early months, though often not completely. Some do and some do not; clearly SGA infants are by no means an aetiologically homogeneous group.

Gruenewald (1963) 30 years ago, and Warshaw (1992) recently, together with Miller (1992), point out that it is inappropriate to diagnose SGA on birth weight alone. They describe what they call symmetrical and asymmetrical intrauterine growth retardation (perhaps the terms proportional/disproportional are more appropriate). In the symmetric type, body weight, body length and head circumference are all proportionately small at birth. In these SGA infants postnatal growth was poor/sluggish.

Asymmetric SGA infants have low weight and length, but normal head circumference. Most of these SGA infants showed postnatal catch-up growth and grew well. A small proportion exhibited some catch-up but, at least by two years, had not yet reached the 5th centile.

An important factor in all SGA infants is whether the soft tissue mass is reduced. Measurement of the above dimensions in all SGA infants at birth is clearly important as is, of course, the calculation of body mass index.

5. Genetic and environmental factors

The Louisville Longitudinal Study of Twins (Falkner & Matheny, 1993) provides evidence of a strong positive correlation between placental weight and birth weight. In monozygous twins, who are phenotypically similar, differences in birth weight and in postnatal growth may be determined by placental weight. It was found that the concentrations of several biochemical and nutritional substances were the same in the placental segments belonging to each twin, but because of the difference in placental weights the absolute amounts were different. Could it be that there is a critical placental mass, below which postnatal growth deficit occurs?

A pair of male monozygous twins from the Louisville Study illustrates many features relevant to this discussion. The smaller twin's (Twin A) birth weight was 52% of that of his larger brother (Twin B). Twin A's placental segment represented only 46% in mass of Twin B's. Most importantly, with both twins born at 39 weeks' gestation, Twin A was a SGA infant.

Fig. 1 shows that Twin A exhibited marked catch-up growth in length during the first nine months to one year in an attempt (hypothetical) to compensate his length deficit of 7.0 cm at birth. Thereafter, both twins grew at approximately the same velocity. However, Twin A never exhibited sufficient catch-up for a long enough period to achieve his co-twin's stature. Both twins started their adolescent growth spurts at the same age (16 years). The within-pair difference was then 8.0 kg in weight and 5.4 cm in stature.

Because head circumference is a good indicator of brain mass (Brandt, 1986; Dobbing & Sands, 1978), it needs to be taken into account in any discussion of IUGR. Placental factors should also always be considered.

6. Reference values for fetal growth

Having noted the absence of any longitudinal growth data on the human fetus, we decided to construct curves of fetal growth. Usher (Usher & McLean, 1969) and Lubchenko (Lubchenko et al., 1970) curves are useful clinically, but as Naeye wrote in 1978: "At present there seems to be no completely valid method available to determine normal fetal growth using measurements from neonates because there is no assurance that prematurely born neonates are normally grown". We have added, after reference to the 'prematurely born' the category of small-for-gestational-age infants. Naeye went on to say: "Ultrasound and other modalities may eventually solve this problem by providing accurate, sequential in utero measurements of normal fetuses".

Regarding fetal length and body weight, was there for example a period of peak length velocity and peak weight velocity comparable to the adolescent velocity peaks of height and weight? Tanner (1950) had suggested that fetal peak growth velocity in length occurs around 20 weeks' gestation, and in weight a good deal later - around 30 weeks' gestation (Fig. 2).

We wanted to verify this and calculated, first, a fetal growth velocity curve for weight from Lubchenko's cross-sectional distance curves (Lubchenko et al., 1970). (We recognised that such calculations disobey good biometric principles, but we were searching for possible indications only.)

Fig. 3 shows indeed that there seems to be a peak weight velocity at 33 weeks. But using the same biometric procedure of calculating velocity from O'Brian & Queenan's (1981) cross-sectional data for fetal femur length, measured ultrasonically, suggests a decelerating velocity curve following a possible peak already at 16 weeks of gestation. They showed too, in a small longitudinal sample, a gradually decelerating curve from 16 to 25 weeks. Deter et al. (1987), in a similar measurement of one fetus, found a peak velocity at 16-19 weeks. Munsick's (1984) cross-sectional leg length data also suggested an acceleration of growth from 9 weeks' gestation to a peak at 15-16 weeks, then a deceleration. After calculating crown-rump velocity from pooled cross-sectional data, a peak velocity at 15-16 weeks appeared (Iffy & Deter, 1975). We started a longitudinal fetal growth study ¹ measuring femur length ultrasonically. The fetal femur can be measured from 15 weeks' gestation to term, and was chosen because from this early gestational age to term it provides a stable measure of linear growth. Measures of postnatal growth outcome were also planned, together with the recording of possible growth factor influences both pre- and postnatally.