The placenta provides the fetus with oxygen and nutrients while removing waste products. Its position is typically assessed during the mid-pregnancy ultrasound scan, usually performed between 18 and 21 weeks. A low-lying placenta occurs when the organ attaches low in the uterine wall, close to the opening of the cervix. This finding is common in the second trimester, but the condition resolves itself in most cases before delivery.
Defining the Diagnosis: Low-Lying Placenta vs. Placenta Previa
The distinction between a low-lying placenta and placenta previa relies on a specific measurement relative to the internal cervical os, the opening to the birth canal. A low-lying placenta is diagnosed when the placental edge is situated within 20 millimeters (or 2 centimeters) of the internal cervical os but does not cover it. This positioning is considered temporary in most instances, as the uterus undergoes significant growth and stretching throughout the pregnancy.
Placenta previa, by contrast, is the diagnosis given when the placenta completely or partially covers the internal cervical os. This presents a higher risk because the placenta is directly blocking the baby’s exit route and is more prone to tearing and bleeding as the cervix begins to thin and open.
The placenta does not physically move, but the rapid expansion of the lower uterine segment pulls the placental attachment site upwards and away from the cervical opening, a process often called “placental migration.” Studies indicate that nearly 90% of placentas initially identified as low-lying in the second trimester will clear the cervix by the time of delivery.
Factors That Increase Risk
The exact cause of low implantation is not fully understood, but several factors increase the likelihood of the condition persisting as placenta previa. A history of previous Cesarean sections is a significant risk factor, as uterine scarring may encourage the placenta to implant lower down. The risk of placenta previa increases linearly with the number of prior C-sections a woman has had.
Advanced maternal age, generally defined as being over 35 years old at the time of delivery, is also associated with a higher incidence. Similarly, having a history of multiple pregnancies or a prior diagnosis of placenta previa increases the susceptibility in a current pregnancy. Assisted reproductive technologies, such as in vitro fertilization (IVF), have been linked to a higher risk of developing a low-lying placenta or placenta previa.
Lifestyle choices, specifically smoking during pregnancy, are known to increase the risk due to the effect of nicotine and carbon monoxide on placental blood flow. Scarring on the uterine lining from previous procedures like a dilation and curettage (D&C) or removal of uterine fibroids can also contribute to abnormal placental implantation.
Monitoring, Management, and Delivery Planning
Once a low-lying placenta is identified during the routine anatomy scan, the standard approach involves a follow-up ultrasound to reassess the position. This repeat scan is typically scheduled around 32 weeks of gestation to confirm if migration has occurred. If the placental edge remains close to the os, a second follow-up scan is usually performed at approximately 36 weeks.
For patients who experience symptoms, such as painless vaginal bleeding in the second or third trimester, management often includes recommendations for “pelvic rest.” This generally means avoiding sexual intercourse and refraining from placing anything into the vagina to prevent disturbing the lower uterine segment. Avoiding strenuous activities and heavy lifting may also be advised to reduce the risk of bleeding.
The final delivery plan depends entirely on the placental position in the late third trimester. If the placenta remains in a previa position, covering the cervical os, a scheduled Cesarean section is mandatory to prevent potentially life-threatening hemorrhage for both mother and baby. However, if the placenta has moved sufficiently, a vaginal delivery may be possible. Current guidelines suggest that a vaginal birth can be safely planned if the placental edge is more than 20 millimeters away from the internal cervical os.
For those with a placental edge distance between 1 millimeter and 20 millimeters, a trial of labor may still be considered after a detailed discussion with the healthcare team, though the chance of an emergency C-section remains elevated. When the distance is less than 10 millimeters, the risk of significant bleeding is higher, making a planned Cesarean delivery the safer option. Even after resolution, women originally diagnosed with a low-lying placenta may still be at a slightly increased risk for postpartum hemorrhage.