Placenta previa happens when the placenta attaches low in the uterus and partially or completely covers the cervix. It occurs in roughly 0.5 to 1% of pregnancies, and the primary drivers are prior uterine scarring, advancing maternal age, and factors that affect how and where a fertilized egg implants. Understanding what raises the risk can help explain why it happens and what it means for the rest of a pregnancy.
How the Placenta Ends Up in the Wrong Spot
In the earliest days of pregnancy, a fertilized egg seeks out a spot on the uterine wall that is rich in oxygen and collagen. Once it finds that environment, its outer cells latch on and begin building the placenta. Normally, this attachment happens in the upper part of the uterus, well away from the cervix.
When the uterine lining has been damaged by surgery, infection, or prior pregnancies, scar tissue creates pockets that are unusually rich in oxygen and collagen. These scarred areas essentially mimic the conditions a fertilized egg is looking for, so it can implant lower than it should. If that implantation happens close enough to the cervix, the growing placenta covers the cervical opening, and placenta previa is the result.
Prior Cesarean Deliveries Are the Biggest Risk Factor
Each cesarean section leaves a scar on the uterus, and the more scars, the higher the risk. A large meta-analysis covering over 170,000 pregnancies found a clear dose-response pattern: women with one prior C-section had about 4.5 times the risk of placenta previa compared to women with no surgical history. Two prior cesareans raised the risk to roughly 7.4 times. Four or more prior cesareans pushed the relative risk to nearly 45 times higher.
This steep escalation matters because the scar from each surgery adds more abnormal tissue to the uterine wall, giving a fertilized egg more low-lying territory to latch onto. It also helps explain why placenta previa rates have risen alongside global cesarean delivery rates over the past few decades.
Maternal Age and Parity
Age plays an independent role. A population-based study found that women 34 and older had two to three times the risk of placenta previa compared to women under 20, with a steady increase at every age bracket in between. The relationship held even after controlling for other factors like prior surgeries and number of pregnancies.
The likely explanation is that the uterine lining changes with age. Years of menstrual cycles, past pregnancies, and natural tissue changes can reduce blood flow to the upper uterus, making the lower segments relatively more attractive for implantation. Having had multiple pregnancies compounds this effect, since each pregnancy remodels the uterine wall.
IVF and Assisted Reproduction
Pregnancies conceived through IVF carry a higher rate of placenta previa than naturally conceived pregnancies. The exact reason is still debated, but the leading theory points to hormonal stimulation rather than the embryo transfer procedure itself. One study found no cases of placenta previa in pregnancies from embryo transfers done during a natural (unstimulated) cycle, suggesting that the fertility drugs used to prepare the uterine lining may alter implantation in ways that favor lower placement.
Earlier speculation focused on the transfer catheter physically depositing the embryo near the cervix, but research on a different technique that places eggs directly into the fallopian tubes showed similar previa rates, making the catheter theory less convincing. Transferring two embryos and using day-5 embryos were both associated with higher previa rates in at least one study, though the mechanisms are not fully understood.
Other Uterine Procedures and Conditions
Any surgery that scars the inside of the uterus can set the stage for placenta previa. This includes procedures to remove fibroids (myomectomy), surgical treatment of abnormal uterine tissue, and dilation and curettage (D&C) after a miscarriage or abortion. The same meta-analysis that quantified cesarean risk also confirmed a significant link between prior abortion and subsequent previa, consistent with the scarring mechanism.
Smoking is another established risk factor. It reduces oxygen delivery to the uterine lining, which may force the placenta to spread over a larger surface area to compensate, increasing the chance that it extends over the cervix. Cocaine use has a similar vascular effect and has been linked to higher previa rates as well.
When Placenta Previa Leads to Deeper Problems
Placenta previa doesn’t just affect where the placenta sits. It also raises the risk of placenta accreta spectrum, a condition where the placenta grows too deeply into the uterine wall and cannot detach normally after delivery. For women with previa and no prior cesarean, accreta occurs in about 3% of cases. But if previa occurs alongside a history of cesarean deliveries, the numbers climb sharply: 11% with one prior C-section, 40% with two, and as high as 67% with five or more. This is one reason prior surgical history matters so much in risk assessment.
Recurrence in Future Pregnancies
If you’ve had placenta previa in one pregnancy, the chance of it happening again is roughly 4.8%, based on population-level data. That’s several times higher than the baseline rate but still means the majority of subsequent pregnancies will not be affected. The recurrence risk is higher if the original contributing factors, like uterine scarring or advancing age, are still present or have worsened.
How Placenta Previa Is Identified
Most cases are discovered during a routine ultrasound around 18 to 20 weeks. At that stage, many placentas that appear low-lying will migrate upward as the uterus expands. A follow-up ultrasound later in pregnancy confirms whether the placenta still covers or sits near the cervix. The key measurement is the distance between the edge of the placenta and the internal opening of the cervix. When the placenta overlaps the opening, a cesarean delivery is necessary. When the edge sits more than 10 mm away, more than two-thirds of women can safely deliver vaginally without increased bleeding risk.
For pregnancies where previa persists and remains stable (no active bleeding or other complications), delivery is typically scheduled between 36 and 37 weeks of gestation. This timing balances the baby’s lung maturity against the risk of unexpected hemorrhage that rises as the cervix begins to thin and open closer to the due date.