Effects of twin pregnancy and periconceptional undernutrition on maternal metabolism, fetal growth and glucose-insulin axis function in ovine pregnancy.

Although twins have lower birthweights than singletons, they may not experience the increased disease risk in adulthood reportedly associated with low birthweight. In contrast, another periconceptional event, maternal undernutrition, does not reduce birthweight but does affect fetal and postnatal physiology in sheep. We therefore studied maternal and fetal metabolism, growth and glucose-insulin axis function in late gestation in twin and singleton sheep pregnancies, either undernourished from 60 days before until 30 days after conception or fed ad libitum. We found that twin-bearing ewes had decreased maternal food intake in late gestation and lower maternal and fetal plasma glucose and insulin levels. Twin fetuses had fewer everted placentomes, grew slower in late gestation, and had a greater insulin response to a glucose challenge, but lesser response to arginine. In contrast, periconceptional undernutrition led to increased maternal food intake and a more rapid fall in maternal glucose levels in response to fasting. Periconceptional undernutrition increased the number of everted placentomes, and abolished the difference in insulin responses to glucose between twins and singletons. Thus, the physiology of twin pregnancy is quite different from that of singleton pregnancy, and is probably determined by a combination of factors acting in both early and late gestation. The inconsistency of the relationships between low birthweight and postnatal disease risk of twins may lie in their very different fetal development. These data suggest that twin pregnancy may be another paradigm of developmental programming, and indicate that twins and singletons must be examined separately in any study of fetal or postnatal physiology.

Cardiovascular adaptations to pregnancy in sheep and effects of periconceptional undernutrition.

The objective of this study was to describe the effects of pregnancy on blood volume and uterine blood flow in sheep, and to test the hypothesis that the effects of periconceptional undernutrition on the late-gestation fetus are mediated by alterations in these parameters. Singleton-bearing ewes that had been undernourished preconception, postconception, both, or neither, underwent estimation of blood volume in mid and late gestation, and measurement of uterine blood flow in late gestation. Seven non-pregnant ewes were also studied. Pregnancy resulted in a 31% greater red cell volume in mid-gestation (21.0+/-1.3 vs 16.1+/-0.8ml/kg, p<0.05), but no significant change in plasma or blood volume. However maternal blood volume was correlated with uterine blood flow (r(2)=0.22, p=0.05) and fetal size (r(2)=0.20, p=0.02). Uterine blood flow was 13% greater in the undernourished groups than controls (1847+/-100 vs 1641+/-79ml/min, p<0.01). The large increase in maternal blood volume integral to a successful human pregnancy was not present in sheep. The increased uterine blood flow after periconceptional undernutrition suggests that nutritional signals before and in early pregnancy influence fetal nutrient supply in late gestation.

A method for assessment of blood volume parameters in pregnant sheep using fluorescein-labelled dextran.

The assessment of blood volume parameters in clinical and research settings has been limited by methods that involve radioactivity, complex assays or are unreliable. We aimed to design a method for measuring blood volume parameters that was non-radioactive, simple, cheap and reliable. We have used a commercially available fluorescein-labelled 250kDa dextran, a large inert molecule, and have measured dilution of this through the intravascular space of pregnant ewes. From this estimation of plasma volume and measured hematocrit, we have calculated blood volume and red cell volume. The blood volume results are 6% lower than those obtained using radiolabelled red cells, but there is no significant difference in red cell volume between methods. The coefficient of variation for repeated measurements of plasma volume measurements is 3.8%. This is a simple, reliable, cheap and non-radioactive method for estimating blood volume parameters in pregnant sheep, and may prove useful in other settings.

Effects of twinning and periconceptional undernutrition on late-gestation hypothalamic-pituitary-adrenal axis function in ovine pregnancy.

The relationships between reduced size at birth, increased activity of the hypothalamic-pituitary-adrenal (HPA) axis, and increased risk of disease in adulthood are well described in singletons but are much less clear in twins. This may be because the physiological processes underlying reduced size at birth are different in singletons and twins. Periconceptional undernutrition can cause altered activity of the fetal and postnatal HPA axis without altering size at birth. However, the independent effects of periconceptional undernutrition and twinning on activity of the maternal and fetal HPA axes are not well described. We therefore studied maternal and fetal HPA axis function during late gestation in twin and singleton sheep pregnancies, either undernourished around conception or fed ad libitum. We found that twinning led to suppressed baseline HPA axis function and decreased adrenal sensitivity to ACTH stimulation but increased fetal pituitary ACTH response both to direct stimulation by CRH (ACTH area under the curve response: 29.7 +/- 2.2 vs. 17.1 +/- 1.6 ng/min x ml, P < 0.01) and to decreased cortisol negative feedback. In contrast, periconceptional undernutrition resulted in a decreased pituitary response (ACTH area under the curve response: 19.4 +/- 1.6 vs. 26.1 +/- 2.2 ng/min x ml, P = 0.02) but no difference in adrenal response. Thus, the HPA axis function of twin sheep fetuses in late gestation is very different from that of control and undernourished singletons. If the HPA axis is an important mediator between fetal adaptations and adult disease, these data may help explain why the relationship between fetal growth and postnatal physiology and disease risk is inconsistent in twins.