Reevaluation of the diagnosis of placenta previa in midtrimester by means of transabdominal, transrectal and transvaginal ultrasonography
Matsuo, K.; Ishihara, K.; Kikuchi, S.
Nihon Sanka Fujinka Gakkai Zasshi 43(6): 603-610
1991
ISSN/ISBN: 0300-9165 PMID: 1856520 Document Number: 378100
The incidence of placenta previa in mid-pregnancy diagnosed by ultrasonography is abnormally high in comparison with its incidence during labor. The cause of this discrepancy has not been fully elucidated. Detection of the cervix is fundamental to the diagnosis of placenta previa. In the present study, the detection rates for three methods, abdominal, rectal and vaginal scanning were compared and the time course of mid-pregnancy placenta previa was traced. The cause of the abovementioned discrepancy was thus elucidated. Rate of cervix detection. In the 12 .apprx. 23 gestational week period, the cervix was successfully identified in 52.0 .apprx. 62.5% of cases by abdominal scanning, and in 85.7 .apprx. 87.5% by transrectal scanning. The uterine isthmus was not identified by transabdominal scanning. Changes in ultrasonographic findings with the passage of time. Transrectal scanning. (1) Transabdominal scans were performed to investigate the placental implantation site in 965 cases in the 16 .apprx. 20 gestational week period. The placenta was found to be low-lying in 64 of them and previa was diagnosed in 12. However, transrectal scans revealed low-lying placentas in 43 (4.5%) of the cases and previa in 4 (0.4%). (2) The diagnosis of placenta previa remained unchanged in all 4 during the 2nd trimester, and the diagnosis was finally confirmed during delivery. (3) The rate at which low-lying placentas returned to the normal site was 79.1% up to the 32nd week of gestation and 11.1% thereafter, representing a significant difference (p < 0.05). 2) Transvaginal scanning. (1) Scans were carried out on 261 cases in the 13 .apprx. 20 gestational week period to determine the site of the placenta. The placenta was found to be low-lying in 18 (6.9%) cases, and the isthmus was identified. (2) By the 22nd week of gestation, when the isthmus cannot be recognized, previa was found in only 2 (0.8%) cases. Thereafter, the diagnosis of previa remained unchanged, and was confirmed during labor. (3) Of the 18 cases with low-lying placentas, the placenta returned to the normal position of 88.9%. It is presumed that in many cases the diagnosis of placenta previa in mid-pregnancy is made on the basis of transabdominal scans, which are unreliable in detecting the cervix. Thus, placenta previa is erroneously diagnosed in cases of low-lying placenta, which, due to the elongation of the lower segment of the uterus, moves easily in relation to the internal os. In other cases, mistaken diagnoses of placenta previa are made before the isthmus becomes dilated.