The placenta plays a major role in the regulation of fetal growth and is known to be affected by maternal nutrient intake during pregnancy [1]. Recent evidence suggests the placenta functions in a sex specific manner [2] . This study investigates the effects of dietary intervention during the periconceptual period and the first trimester upon placental development and fetal growth trajectory. 

Primiparous heifers (n=360) were selected from a range population. Sixty days prior to artificial insemination heifers were randomly assigned to two equal groups and individually fed a high (14%CP) or low (7%CP) protein pellet diet. They were inseminated with semen from the same bull. 18 dpc each high or low group was again split into high (H) or low (L) protein diet treatment for the first trimester yielding four treatment groups (HH, HL, LH, LL). Diet treatments ceased at 98 dpc. Fetuses (n= 48) and placentae were excised at 98 dpc, weighed, measured and dissected. The remaining heifers (n=64) were taken to term. 

At gd 98, male placentae were more closely correlated to the weight of the fetus than the female placenta (Coef.: 0.573; p<0.05 vs Coef: 0.378; p<0.1). At term, placental measures were correlated in the male fetuses in the LL (Coef.: 0.187; p<0.05) and HH (Coef.: 0.258; p<0.05) groups. However, in the LH and HL, the normal correlation between placental weight and birth weight was lost, as it was in all groups for the female. At term, there was no relationship between sex and birth weight (p= 0.379) whereas male calves are usually significantly larger than female [3]. Combined, these results suggest the diet perturbation in early gestation has altered the both the pattern of placental development and the feto-placental ratio. We suggest that the male feto-placental unit adjusts to perturbation early in gestation unlike the female which is able to adjust mid to late term[4]. 

Acknowledgements: We are indebted to S.Kidman and Co., Ridley Agriproducts and ARC for funding this research