Variety in placental shape: when does it originate?

OBJECTIVES: Observational and empirical evidence suggest that the average placental shape is round with a centrally inserted umbilical cord. Yet variability of shape is common. When in pregnancy do shape and cord insertion variations originate?
MATERIALS AND METHODS: Placental measures from published datasets obtained ultrasonographically at 11-14 weeks and/or at term were correlated.
RESULTS: Significant correlations were found between the normalized distance of cord insertion to the margin at 11-14 weeks with the same quantity at delivery (r = 0.509, p < 0.0001). First trimester cord marginality was not correlated with two measures of roundness of the delivered placenta (p = 0.448, and p = 0.812). There was a strong correlation between delivered placental thickness and first trimester cord marginality (r = -0.368, p = 0.009). There was a significant relationship between the cord marginality at 11-14 weeks and the mean chorionic vascular density at delivery (r = -0.287, p = 0.015). Placental position in the uterine cavity influences cord marginality at delivery. Modeling suggests that placental growth in the first trimester is non-round. Placental shape at 11-14 weeks is found to be irregular. This irregularity is not correlated with the roundness of the delivered placenta. Both empirically, and in the context of IVF pregnancies, deformation of the vasculogenic zone yields a bi-lobate placental shape.
CONCLUSIONS: Our findings strongly support the hypothesis that abnormal cord insertion and a multi-lobate shape result from early influences on the placental growth, such as the shape of the vasculogenic zone, or placental position in the uterus, rather than trophotropism later in pregnancy.