Umbilical blood flow during pregnancy: evidence for decreasing placental perfusion.

OBJECTIVE: The present study was designed to determine the relation of umbilical venous blood flow (UmbBF) to fetal weight (FW) at different times in late pregnancy, and to assess fetal O2 supply near term. STUDY
DESIGN: In 46 pregnant women, UmbBF was calculated just before delivery using the product of flow velocity and the cross section of the umbilical vein determined by pulsed Doppler technique and measuring of the diameter of the vessel, respectively. Based on the gestational age at delivery (range, 29-42 weeks), infants were divided into a preterm group (PT; < or = 36 weeks, n = 13) and a full-term group (FT; > 36 weeks, n = 33). Blood gas, pH, and hemoglobin analysis was performed in specimens of umbilical venous and arterial blood obtained after delivery.
RESULTS: UmbBF was higher in FT infants (515 +/- 125 mL/min, mean +/- standard deviation) than in PT infants (423 +/- 120 mL/min; P < .05). This was associated with a larger increase in umbilical vein diameter: FT 8.8 +/- .7 mm, PT 8.1 +/- .6 mm (P < .01). Partial pressure of O2 (pO2) did not differ significantly between FT and PT; the correlation of pO2 with gestational age showed a weak decrease (P < .05). Hemoglobin was elevated in FT (P < .01), whereas O2 content remained constant in PT and FT. The ratio UmbBF/FW was considerably reduced in FT [154 +/- 37 (mL/min)/kg], relative to PT [221 +/- 37 (mL/min)/kg; P < .001], and was accompanied by a marked reduction of O2 transport capacity: FT 17.6 +/- 6.7 and PT 26.6 +/- 9.2 (mL/min)/kg (P < .01).
CONCLUSION: Due to the growth of the umbilical vein, UmbBF increases over the last weeks of gestation. The ratio UmbBF/FW is reduced in FT. Despite a constant O2 content, the continuous weight-related decrease in UmbBF results in a reduction of the fetal O2 transport capacity per unit that may contribute to an adverse intrauterine environment at the end of gestation, especially in postterm pregnancies.