Evaluating and management of low-lying placenta or placenta previa on second-trimester ultrasound -

Evaluating and management of low-lying placenta or placenta previa on second-trimester ultrasound - - ModernMedicine

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A. Various definitions of low-lying placenta and placenta previa have been used in the medical literature,^1,2 but most researchers recognize the increased risks associated with a placenta located within 20 mm of the os and especially overlapping the os.^1-3 For the purposes of this article and to standardize the definitions, we have defined placenta previa as one that covers the internal os of the cervix, and low-lying placenta as one of which the edge is 1 mm to 20 mm of the internal os. A distance of more than 20 mm from the lower placental edge and cervical internal os in the second trimester is a normal finding that does not represent a low-lying placenta and does not require follow-up.

Figure 1 Transabdominal ultrasound of a placenta previa at 20 weeks. Neither the cervical internal os nor the lower placental edge can be adequately visualized. Shadowing prevents an adequate view of the cervix. P indicates placenta; B, bladder; C, cervix.

The widespread use of ultrasound for screening purposes in the second trimester has led to the frequent finding of placenta previa or low-lying placenta in otherwise asymptomatic women.⁴ Incidence of this finding depends on the sonographic method used.⁵ With transabdominal sonography, the incidence is about 5% to 20%.^4,6 Frequently, however, the finding of a second-trimester low-lying placenta or placenta previa is a false positive, partly because the bladder filling used in transabdominal ultrasound may exaggerate the length of the cervix and compress the lower uterine segment.⁷ Also, in at least one-half of all cases, the exact locations of the internal os and lower placental edge are not accurately seen on transabdominal sonography (Figure 1).⁵ The true diagnosis of placenta previa or low-lying placenta should be made by transvaginal ultrasound (TVU). TVU is safe, accurate, and well tolerated.^8-10 With TVU, the incidence of placenta previa in the second trimester is about 1% to 4%.^10-12

What further evaluation is needed to confirm the diagnosis and assess prognosis?

In cases of suspected placenta previa or low-lying placenta, the first step in determining accurate placental position in relation to the cervix is to perform a TVU on a patient whose bladder is empty. In at least one-quarter of all cases of placenta previa or low-lying placenta suspected on the basis of transabdominal sonography in the second trimester, the diagnosis will change after TVU is performed.⁵ Those patients who do not have placenta previa or low-lying placenta can then be reassured and will require no further follow-up.

Figure 2 Transvaginal sonogram of the patient in Figure 1. The internal os of the cervix can be clearly seen (arrow), as can the lower placental edge, which just overlies the internal os. This is a posterior complete placenta previa. Because the degree of overlap is minimal, this placenta is likely to no longer be a placenta previa at term. P indicates placenta; C, cervix.

An alternative option is to delay performing TVU and re-evaluate placental location at the 28- to 32-week follow-up abdominal ultrasound. If the placenta is clearly no longer within 20 mm of the internal os, TVU is not necessary. Alternately, if the placenta appears low-lying or previa by transabdominal ultrasound, TVU should be performed (Figure 2).

Table: Likelihood of persistence of placenta previa or low-lying placenta at delivery

Approximately 80% of women with a placenta previa or low-lying placenta on TVU in the second trimester will no longer have the condition at term.¹⁰ When a placenta is shown by second-trimester TVU to be low-lying or previa, further prognostic information can be gained by assessing its location (ie, distance either between the lower placental edge and internal os or the distance of overlap), the gestational age at which it was detected, and any history of prior cesarean delivery (Table).^11,13-17 For example, some studies report that only women who have a placenta that actually overlaps the internal os at 15 to 24 weeks had previa at delivery.^11,12,15 The earlier the previa or low-lying placenta is detected, the less chance it will persist at delivery.¹³ However, the presence of a prior cesarean delivery increases the chance of persistence (Table).^13-16

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