Placenta Previa

July 1, 2013
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By John C. Hobbins, MD, Professor, Department of Obstetrics and Gynecology, University of Colorado School of Medicine, Aurora. Dr. Hobbins reports no financial relationships relevant to this field of study.

Synopsis: A new study shows that the vast majority of low-lying placentas or marginal previas noted in second trimester patients resolved by term. However, there was a doubling of the rate of postpartum hemorrhage in these patients even if the placentas had migrated away from the cervix.

Source: Osmundson SS, et al. Second trimester placental location and postpartum hemorrhage. J Ultrasound Med 2013;32:631-636.

With the recent emphasis on early prenatal diagnosis, most patients today are having ultrasound examinations in the first trimester for nuchal translucency testing followed by a second trimester evaluation of fetal anatomy. Although the focus has been predominantly on the fetus, every scan includes an assessment of placental location. Sometimes a low-lying placenta or even a placenta previa is discovered. Since the meaning of the low-lying or marginal previa is unclear, counseling and management have varied widely among clinicians.

A group from Northwestern University recently published a study that primarily dealt with whether a low- lying placenta found between 18 and 24 weeks of gestation carries an increased risk for postpartum hemorrhage.1 The study also revealed some other spinoff findings that may help to clarify the meaning of this ultrasound finding.

Over a period of 6 months, 282 patients with adequate follow-up data were diagnosed as having low-lying placentas between 18 and 24 weeks of gestation (placental edge was 0.1-2.5 cm from the endocervix). Another 61 patients with marginal previas (touching, but not overlapping the endocervix) and 30 patients with complete previas (covering the cervix) were included. “Resolution” meant that within 28 days of delivery the placental edge was noted to be > 2.5 cm from the endocervix. Persistent low-lying placentas were those that remained < 2.5 cm from the cervical os prior to delivery.

The diagnosis of postpartum hemorrhage was made if the blood loss was estimated to be > 500 cc after vaginal delivery or > 1000 cc after cesarean section. A separate group of 410 women with placentas that were > 2.5 cm from the cervix at 18-24 weeks were used as controls.

Low-lying placentas resolved in 98% of patients, while marginals and complete previas resolved in 89% and 59% of cases, respectively. Antepartum bleeding occurred in 32% vs 0.5% (P = 0.004), postpartum hemorrhage in 12.4% vs 4.9% (P = 0.001), and need for uterotonics 11.0% vs 6.1%, (P = 0.01) compared with controls. Statistically significant differences remained in all categories even if the low-lying, marginal, or complete previas had resolved by late gestation.


The term “placenta previa” carries a stigma that puts into play lifestyle proscriptions such as no intercourse, no physical exertion, no travel, etc., and the diagnosis often leaves patients with the feeling that they are sitting on a time bomb. Now it should be clear that the second trimester finding of a placenta that is close to, if not just covering, the cervix carries far less meaning than if found toward term. The authors have again uncovered what others have noted: that only a few of these placentas will remain close to the cervix as pregnancy progresses.

Why this happens is still a matter of conjecture but one explanation is that the lower uterine segment lengthens out, pulling the placenta away from the cervix. Another theory is that the lower uterine segment is a less hospitable place for the placenta to implant, and the placenta, in an effort to seek a better environment on higher ground, atrophies near the cervix while proliferating in its upper portion — an activity called “trophotrophism.”

Another question that emerges in patients with lowlying placentas at term is whether they can have vaginal deliveries. For years, the definition of a placenta previa included placentas whose lower edge extended down to within 2 cm of the endocervix. Also, placenta previa essentially meant delivery by cesarean section. Now it is quite clear that many of these patients can deliver vaginally. For example, Vergani et al noted that patients with placentas between 1.0-2.0 cm of the cervical os had only a 31% chance of having a cesarean section and a 3% chance of antepartum bleeding.2 If placentas were between 0.1-1.0 cm from the cervix, the cesarean section rate was 75% and the rate of antepartum bleeding was 29%. These results suggest that more than two-thirds of patients in the 1-2 cm range, previously considered by standard definition to have placenta previas, could actually have a successful vaginal delivery. Even one out of four of those with placentas within 1 cm of the endocervix could have vaginal deliveries, but at a higher rate of antepartum bleeding.

The major thrust of the featured study was to determine how often a finding in the second trimester of a low-lying placenta is associated with postpartum bleeding.1 There was a clear association, even in patients where there was resolution of the finding in the third trimester. In these cases, the odds ratio for hemorrhage was 2.7 (95% confidence interval [CI], 1.46-5.07) and need for uterotonic agents was 2.1 (95% CI, 1.24-3.84) compared with controls.

Why would this be? We assume that most postpartum bleeding comes from the exposed vascular surface left by the vacated placenta and this is rectified by tamponade created by strong generalized uterine contraction. Unfortunately, the lower segment contracts less well than the upper segment, and often help is needed from oxytocin and other forms of uterine stimulation. This study simply alerts the clinician to this possibility. However, there is no ready explanation as to why the lower uterine segment site, vacated by the long-gone placenta in the “resolved” group, would be involved in the bleeding after delivery.

Below are some suggestions for dealing with the inadvertent finding in the second trimester of a low-lying or marginal placenta:

1. In the first and second trimester, the term “placenta previa” should be rarely used and the diagnosis should be reserved only for those patients with vaginal bleeding and placentas that are clearly overlapping the cervix.

2. If the patient has had no bleeding, there is no evidence to indicate that she should alter her lifestyle.

3. Have the patient return after 32 weeks to document resolution.

4. Apprise motivated patients wishing to avoid cesarean section, whose placental edges are still 1.0-2.0 cm from the cervix by term, that they have about a 70% chance of delivering vaginally.

5. Be prepared for a greater chance of postpartum hemorrhage, even if the placenta is no longer in the vicinity of the cervix before delivery.


1. Osmundson SS, et al. J Ultrasound Med 2013;32:631-36.

2. Vergani P, et al. Am J Obstet Gynecol 2009;201:266.e1-5.