Intrauterine growth restriction (IUGR) and low birth weight (LBW) affect 5-15% of babies and increase their risk of perinatal death and poor health in later life (1, 2). Rates of LBW increase in multiple pregnancies; 5% singletons, 54% twins and 98% higher-order multiples were LBW for 2015 Australian pregnancies (1), likely due to restricted implantation and competition for nutrients. Spontaneous variation in litter size is confounded by maternal factors. We therefore aimed to utilise embryo transfer to produce murine IUGR due to large litter size. CBAF1 embryos were collected at gestation day 0.5 (GD0.5) and 6, 8, 10 or 12 embryos transferred into each uterine horn of pseudopregnant CD1 mice (n=32, 26 completed). At GD18.5, umbilical (3-4 fetuses/litter) and uterine blood flows were analysed by Doppler ultrasound prior to post-mortem. Fetal morphometry has been measured in 16 litters to date. Litter size (viable pups, R2=0.334, P=0.002) increased linearly as more embryos were transferred. At GD17.5, litter average placental weight (R2=0.456, P<0.001), and fetal weight (R2=0.404, P<0.001), abdominal circumference (R2=0.452, P=0.004) and head width (R2=0.255, P=0.046), but not crown-rump length (R2=0.133, P=0.166) correlated negatively with litter size (Figure). Fetuses from larger litters had lower weight:length ratios (R2=0.332, P=0.020) and tended to have increased head width:abdominal circumference ratios (R2=0.214, P=0.071). In preliminary analysis (data from 7 litters), litter average umbilical and uterine arterial resistance and pulsatility indices were not correlated with litter size or litter average fetal weight.
Increased litter size in embryo transfer-generated pregnancies induces an IUGR phenotype, including lighter placentas and fetuses, which are thinner with evidence of head sparing. Analysis of Doppler blood flow parameters for individual fetuses is currently in progress. We will now utilise this approach in combination with murine genetic models to identify mechanisms and test new intervention strategies to prevent IUGR due to chronic restriction.