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Recognizing Placenta Accreta Before Delivery
Abstract & Commentary
By John C. Hobbins, MD, Professor and Chief of Obstetrics, University of Colorado Health Sciences Center, Denver, is Associate Editor for OB/GYN Clinical Alert.
Dr. Hobbins reports no financial relationships to this field of study.
Synopsis: Patients with submucous fibroids and history of hysteroscopic procedures might benefit from an ultrasound evaluation to look for signs of placenta accreta.
Source: Al-Serehi A, et al. An association with fibroids and Ascherman syndrome. J Ultrasound Med 2008;27:1623-1628.
Placenta accreta can represent a real clinical conundrum, especially if it is unrecognized before delivery. Therefore, clinicians have been justifiably sensitized to its possible presence in patients who have the greatest risk for it — those who have had a previous cesarean section. However, in this recent report, two cases were presented that represent two variations on the placenta accreta theme.
The first case involved a 40-year-old woman who had had a myoma resected, as well as lysis of adhesions, by hysteroscopy. The patient had had one previous cesarean section. She became pregnant through in vitro fertilization with donor eggs from a 30-year-old. Not surprisingly, she was found early on to have twins. Two more 2 cm fibroids were noted in the anterior wall of the uterus at the time of her first examination. However, the placenta was on the posterior wall. At 34 weeks she had severe vaginal bleeding requiring an emergency cesarean section. After delivery of the baby, a supracervical hysterectomy was done to stop the bleeding. Pathology showed a complete placenta accreta. With the benefit of hindsight, the authors re-evaluated the last ultrasound scan at 32 weeks and found none of the typical criteria for this condition.
The second case involved a patient who was noted at 18 weeks to have a complex mass in the base of the placenta with low resistance flow around it. At first, it was contiguous with the uterine wall, but later this was not clear. It did not grow, and the pregnancy continued uneventfully until term, when she delivered spontaneously without problems. The "tumor" came out intact with the placenta. Pathology showed it to be a submucous fibroid and the trophoblast had worked its way around the base of the fibroid to completely separate it from the uterine wall.
These two cases show us that there may be other predisposing factors to placenta accreta, in addition to a previous cesarean section with placenta previa — namely, previous hysteroscopic surgery and fibroids.
First, since the best way to avoid disaster is to be forewarned, let's touch upon the typical ultrasound findings with accreta and the accuracy with which ultrasound techniques can predict this condition. Finberg first described criteria correlating with accreta, which included thinning of the interface between the bladder and the uterine wall and a loss of the "clear space" between the placenta in the uterine wall.1 He also described what we have found to be the most consistent predictor of accreta, a Swiss cheese-appearing tissue near the basal plate, with swirling lacunar flow within the echo-spared areas. Comstock recently has added the presence of color Doppler flow in the affected area, "beating" at a fetal rate.2 Using these criteria, in perhaps the largest accumulation of cases, Warshak et al found the diagnostic sensitivity of ultrasound to be 82%, with a specificity of 96%.3
MRI has also been used to diagnose this condition, and the results have been no better than with ultrasound. However, this method has been found to be helpful in patients with equivocal ultrasound findings. Our experience is that ultrasound has outperformed MRI, since the results with the MRI often are not definitive.
An interesting, but little-known fact is that 50% of accretas are associated with elevations of second trimester maternal serum alpha-fetoprotein (MSAFP) well above 2.0 MoM.
Fibroids are becoming a more common accompaniment to pregnancy because more patients of advanced maternal age are now becoming pregnant. However, it is unclear what real effect they have on pregnancy. There are two studies showing a modest increase in seemingly every complication known to pregnancy. However, although I have not been able to quantify this, my feeling is that the overwhelming majority of patients with fibroids sail through pregnancy without a hint of a problem. We do know that they grow most rapidly in the first trimester and taper off thereafter to a point where they rarely increase in size after 20 weeks. Also, they never win a competition for the blood supply to the fetus.
The two cases above should not cause us to adjust our management of patients with fibroids to include, for example, an expensive MRI. However, here are some suggestions to diminish the chances of being blindsided by placenta accreta.
Patients at greatest risk include:
Those whose risk is on a lower scale include:
If the patient has either criteria 1 or 2, and, theoretically, 3, she would benefit, at least, from a special ultrasound scan after 32 weeks to look for signs of accreta, and if the results are equivocal, an MRI could be useful. With criteria 4 and 5, a comprehensive ultrasound evaluation of the placental interface would only be in order for patients with elevated MSAFP.4,5
(Addendum: Although the aim of this review is to deal with the diagnosis of placenta accreta, there is now a trend in some centers to adopt an approach of watchful waiting in patients wishing to preserve reproductive function, by leaving the placenta intact. There is no consensus as to whether to use methotrexate to shrink the tissue, but one can use Doppler flow studies of the uterine cavity to determine the ideal time for later removal of the tissue by D&C.)