Placenta Previa, or low lying placenta, is a condition where the placenta either partially or completely covers the cervix. It affects approximately 5 in every 1000 pregnancies globally, with the highest prevalence seen in Asian women, suggesting a genetic predisposition towards the condition. Alternative factors that increase the risk of placenta previa include uterine scarring, possibly as a result of pre-pregnancy surgery, such as fibroid removal; and previous caesarean section (C-section). The more C-sections a woman has had, the greater her risk of developing placenta previa; 1 prior C-section has an odds ratio of 4.5, however, 4 prior C-sections increases the ratio to 44.9.
The association between surgical scarring of the uterus and placenta previa appears to be driven by the placenta preferentially implanting at these sites. Fortunately, in 90% of cases, as the pregnancy progresses, the placenta migrates to an area with a richer blood supply. The earlier in a pregnancy that placenta previa is identified, the more likely it is to self-resolve.
What are the risks associated with placenta previa?
The greatest risk to a mother who develops placenta previa is prolonged, heavy vaginal bleeding. As well as causing emotional distress, this can have serious medical repercussions and, in cases of severe haemorrhage, necessitate a blood transfusion. Bleeding can occur before, during or after birth. If it happens during pregnancy, it is likely that bed rest will be recommended for the remainder of the pregnancy. There is no substantial evidence to support hospitalisation; however, if there are repeated episodes of bleeding, frequent hospital trips are likely. If the bleeding becomes heavy and cannot be managed, it may be necessary for the mother to undergo an emergency C-section. All females diagnosed with placenta previa should be treated at high-risk labour and delivery units, with experienced staff on hand. It is difficult to ascertain precisely if and when bleeds will occur, and how serious they will be. Appropriate preparation for every eventuality is key.
Do all women with placenta previa require a C-section?
The short answer is no, not all women diagnosed with placenta previa will require a C-section. It largely depends on how much of the cervix is covered.
Placenta previa is best diagnosed using transvaginal ultrasound, as this enables measurement to the nearest mm. The doctor will use these findings to categorise the condition as complete, partial or marginal. Marginal cases resolve themselves up to 98% of the time, meaning that in the majority of instances (assuming there are no additional complications) a vaginal birth will be possible.
Women who have complete placenta previa will be advised to undergo an elective C-section. As the body prepares itself for birth, the cervix naturally starts to open. If this happens whilst the placenta is lying across the opening, the placenta may start to split, causing internal bleeding, which can be very dangerous for the mother. There is also a risk of extensive haemorrhaging during and after birth and so, in cases of complete placenta previa, a vaginal birth will be considered too risky and a C-section will be scheduled. Ideally, this will occur after 36 weeks, which is the point at which the baby’s lungs are considered to be fully developed. If an earlier delivery becomes necessary, corticosteroid injections will likely be offered, to speed up lung maturation.
Partial placenta previa is less definitive and the decision as to whether or not to undergo a C-section will likely involve significant consultation and evaluation between doctor and patient. Original guidelines stated that only if the edge of the placenta was more than 20 mm from the cervical opening (the internal os, also known as the orifice of the uterus) should natural labour be attempted. If the edge of the placenta is within 10 mm of the internal os, most doctors will still recommend a C-section over a vaginal delivery, as the risk of bleeding remains high. The area of uncertainty is when the placenta lies between 11 and 20 mm from the internal os. One study found that of the women who fell into this category and opted to attempt a natural delivery, more than 90% delivered safely without significant bleeding.
One thing to bear in mind when making a decision is that elective C-sections are a far safer option than emergency C-sections. Ultimately, the health of the mother and unborn baby comes first and a good doctor will advise accordingly.
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- Al Wadi, K, et al. “Evaluating the Safety of Labour in Women With a Placental Edge 11 to 20 Mm From the Internal Cervical Os .” Journal of Obstetrics and Gynaecology, Canada, vol. 36, no. 8, Aug. 2014, pp. 674–677., doi:10.1016/S1701-2163(15)30508-9.
- Eichelberger, K Y, et al. “Placenta Previa in the Second Trimester: Sonographic and Clinical Factors Associated with Its Resolution.” American Journal of Perinatology, vol. 28, no. 9, Oct. 2011, pp. 735–739., doi:10.1055/s-0031-1280853.
- Johnson, S. “Low-Lying Placenta (Placenta Previa).” Healthline, 22 Sept. 2016, www.healthline.com/health/placenta-previa.
- Rowe, T. “Placenta Previa.” Journal of Obstetrics and Gynaecology, Canada, vol. 36, no. 8, Aug. 2014, pp. 667–668., doi:10.1016/S1701-2163(15)30503-X.