When discussing vaginal delivery, people often first consider its benefits—aiding newborns in establishing autonomous breathing and enabling mothers to recover more quickly postpartum. However, not every mother is suitable for vaginal delivery, such as those with low-lying placentas. Why is this? Let's explore.
Ultrasound (especially transvaginal ultrasound) can more accurately determine the relationship between the placental edge and the internal cervical os.
After 28 weeks of gestation, if the placenta attaches to the lower uterine segment—or if its lower edge reaches or covers the internal cervical os—and its position is below the fetal presenting part, it is termed placenta previa. Placenta previa is a serious late-pregnancy complication and a common cause of severe bleeding during late pregnancy and delivery. When diagnosing placenta previa via ultrasound, gestational age must be carefully noted.During the first trimester (before 28 weeks), the placenta occupies half the uterine wall area, increasing the likelihood of proximity to or coverage of the internal cervical os. By the second trimester (after 28 weeks), the placenta occupies only one-third to one-quarter of the uterine wall area. The formation and expansion of the lower uterine segment increase the distance between the internal cervical os and the placental margin. Consequently, a placenta initially located in the lower uterine segment may migrate upward with the uterine body, transforming into a normally positioned placenta.
A low-lying placenta increases bleeding risk and is unsuitable for vaginal delivery.
Therefore, many scholars believe that if placenta previa is detected during mid-pregnancy ultrasound, it should not be diagnosed as "placenta previa" but rather as "placenta previa status." Ultrasound follow-ups should be conducted approximately every 4 weeks. If vaginal bleeding occurs, an earlier ultrasound is necessary to allow the doctor to make timely and appropriate decisions.If diagnosed with placenta previa but without vaginal bleeding or only minimal bleeding, the pregnancy should be extended to 36 weeks whenever possible while ensuring maternal safety.After 35 weeks of gestation, the frequency of physiological uterine contractions increases, leading to a significant rise in bleeding rates for placenta previa and heightened risks for both mother and fetus. Therefore, at 36 weeks, healthcare providers will assess fetal lung maturity and may opt for a timely cesarean section to terminate the pregnancy.
Cesarean delivery allows for rapid fetal extraction, prompt placental separation, and the use of uterotonic agents to enhance uterine contractions. Under direct visualization, the bleeding site at the placental separation in the lower uterine segment can be closely monitored, enabling precise application of medication or surgical intervention to control hemorrhage and prevent intrapartum or postpartum hemorrhage.
Editor's Note: Every mother hopes for a natural vaginal delivery to bring a healthy baby into the world. However, vaginal delivery isn't suitable for every mother. For women with a low-lying placenta, the risk of bleeding during delivery increases, leading doctors to favor cesarean delivery to reduce complications.