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Causes of Placenta Previa

Placenta Previa is defined as implantation of the placenta either partially or completely over the lower uterine segment. The lower uterine segment is the lower part of the uterus adjoining the cervix which is produced by thinning of the uterine wall with growth of the uterus during later stages of pregnancy. Hence this diagnosis is most accurately made during the second and third trimesters when the lower uterine segment is well formed. The word previa refers to the position of the placenta in relation to the presenting part of the baby.

This condition accounts for around a third of all cases of bleeding during pregnancy, with the other third being caused by abruption and another third due to unexplained causes. The incidence of this condition is around 1 in 100, but these statistics are derived from hospital cases which only reflect the tip of the ice berg.

The exact reasons for the implantation of the placenta in the lower uterine segment are not clearly known. But the following theories have been postulated:-

· Dropping down theory: It is believed that poor decidual reaction in the upper part of the uterus may force the fertilised ovum to drop and implant itself to the lower part of the uterus.

· Lesser degrees of this condition can be explained by the persistence of chorionic activity in decidua capsularis, which leads to formation of a large capsular placenta. This large capsular placenta on contact with the decidua vera of lower segment implants in it.

· Defective decidua can result in compensatory proliferation of the placenta. This leads to a large placenta which can encroach into the lower segment.

· A very big placenta (as in twin pregnancy) can encroach into the lower segment of the uterus.

Some of the conditions which predispose to placenta previa are as follows:-

· Multiparity: This is a very important predisposing factor. It has been observed that rates of this condition increases with parity (Parity refers to the number of times a woman has given birth).

· Increased maternal age: This is another important contributing factor. Women over the age of 35 years have the greatest chance of developing placenta previa.

· Previous C Section: Previous C section or any other surgery to the uterus which leads to a scar is known to increase the risk of placenta previa by around two times.

· Placental size and abnormalities: As mentioned above placental size affects the incidence of placenta previa. Apart from this many placental abnormalities like succenturiate lobes also lead to increased risk.

Source by Sriram Ravichandran

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