Inter-twin hemoglobin difference at birth in uncomplicated monochorionic twins in relation to the size of the placental anastomoses.

OBJECTIVE

To evaluate the relation between Hb levels and the size of the placental anastomoses.

METHODS

We performed a retrospective review of all uncomplicated MC twins delivered vaginally at our center from 2002 to 2017. Hb levels at birth and on day 2 were retrieved. All MC placentas were routinely injected with colored dye and high-resolution pictures were taken for computer-based analysis. We measured the size of arterio-arterial (AA) and veno-venous (VV) anastomoses as well as the total venous size, defined as the sum of the diameter of first generation of placental veins within 5 cm of each cord insertion. We assessed the relation between Hb levels and placental angioarchitecture.

RESULTS

A total of 170 MC twin pairs were analyzed. Median Hb level in twin 1 was significantly lower than that in twin 2 both at birth (16.0 versus 17.4 g/dl, P = 0.02) and on day 2 (14.6 versus 18.1 g/dl, P = 0.000000188). Inter-twin Hb difference on day 2 was positively correlated with the size of AA anastomoses (Spearman r = 0.25, 95% CI 0.04-0.43, P = 0.0161). The diameter of AA anastomoses was positively related to the total size of veins connecting to AA anastomoses in the placental territory of twin 2 (Spearman r = 0.55, 95% CI 0.41-0.66, P = 0.0001).

DISCUSSION: MC twins with larger AA anastomoses have higher Hb differences at birth. Higher Hb levels in second born twins may partly be due to increased placento-fetal transfusion through larger placental vessels.