Placental Implantation as the Cause of Breech Presentation

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  • 骨盤位の成因としての胎盤付着部の意義

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The ultrasonographic study of 5294 singleton pregnancies between the 16 and 41 gestational weeks and 47 breech presentations at term revealed the following : 1) The placenta was found in the mid-anterior or mid-posterior region of the uterus in 60.9 to 74.0%.In 12.8 to 18.7% of cases, the placenta was implanted in the fundal region before gestational week 28, but thereafter it was found in the cornual region in 13.8 to 19.1%. 2) Of 47 cases with breech presentation at term, the placenta was implanted in the cornual region in 28, in the fundus in 7, in the lateral in 6, and in the mid-anterior or mid-posterior in another 6. 3) In gestational weeks from 16 to19, the breech presentation was found in 38% when the placenta was implanted in the cornual region, in 50% when it was implanted in the lateral region, 52.8% in the fundus, and 47.2% in the middle, with no significant difference in the incidence of occurrence. However in gestational weeks from 36 to 41, the incidence was 20.2, 6.4, 9.0 and 1.1% respectively and the spontaneous version rate was significantly higher when the placenta was implanted in the middle region than in others(p< 0.01). 4) The author has concluded that the breech presentation in single pregnancy is caused by the placental position ; when it markedly indents and changes the inverted pear shape of the amniotic cavity, the spontaneous fetal cephalic version is inhibited. Furthermore, the cornual implantation of the placenta causes the fundal pole of the amniotic sac to have a smaller capacity than the lower segment. Because the fetal head is smaller then its breech and legs, it would be natural for the breech and legs to accommodate themselves to a larger space, that is, to the lower uterine segment-causing the breech presentation.

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