Opening of the uterine isthmus at 11-13weeks' gestation is not related to developmental abnormalities of the placenta

Objectives: To clarify whether openning of the uterine isthmus between 11 and 13. weeks' gestation adversely affects the placental development. Methods: The uterine cervix and isthmus lengths were measured prospectively using transabdominal ultrasound at 11 to 13. +. 6. weeks' gestation. Following delivery, the pregnancy and delivery course were reviewed. The associations between the conditions of the uterine isthmus during the first trimester and the perinatal complications associated with placental abnormalities were analyzed. Results: A total of 653 cases were analyzed. The isthmus had been completely opened (isthmus length. = 0. mm) in 15.9%, 21.6% and 24.1% of the cases at 11, 12 and 13. weeks' gestation. The frequencies of perinatal complications in the cases whose isthmus had been opened (cases) and the other cases (controls) were as follows; 1.4% and 1.0% (ns) for placenta previa, 1.4% and 1.2% (ns) for abnormal cord insertion, 3.5% and 4.3% (ns) for pre-eclampsia, 9.9% and 9.0% (ns) for small for gestational age, and 12.0% and 7.6% (ns) for preterm delivery, respectively. Conclusion: Early opening of the uterine isthmus at 11 to 13. weeks' gestation does not appear to adversely affect either the placental development or the course of pregnancy.