In contrast to the singletons, in control twin pregnancies, there was no effect of maternal weight gain in the periconceptional period on any measure of uteroplacental growth, and there was also no relationship between placental and fetal weight. There were direct relationships, however, between placental and fetal weights in both control and PCUN singleton pregnancies. In control, but not PCUN ewes carrying singleton pregnancies, there were direct relationships between maternal weight gain during the periconceptional period and uteroplacental weights at 53–56 days' gestation. during the period of maximal placental growth. We have investigated the effect of periconceptional undernutrition in the ewe (control n = 24, restricted at 70% of control feed allowance, PCUN n = 21) from 45 days prior to mating until 7 days after mating on placental and fetal weight and on placental histology in singleton and twin pregnancies at 53–56 days' gestation, i.e. We have tested the hypotheses that there are relationships between maternal weight or body condition at the time of conception and feto-placental growth during the first 55 days of pregnancy, and that periconceptional undernutrition has a differential effect on these relationships in singleton and twin pregnancies. Recent studies in the sheep have shown that maternal undernutrition during the periconceptional period, when the nutrient demands of the embryo are minimal, can alter the subsequent development of the metabolic, endocrine and cardiovascular systems and that these effects may, in part, depend on embryo number.
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