Placenta previa is defined as implantation of the placenta either partially or completely on the lower uterine segment. The lower uterine segment is the lower part of the uterus adjoining the cervix that is caused by the thinning of the uterine wall with the growth of the uterus during the later stages of pregnancy. Therefore, this diagnosis is most accurately made during the second and third trimesters when the lower uterine segment is well formed. The word anterior refers to the position of the placenta in relation to the presenting part of the baby.
This condition accounts for about a third of all cases of bleeding during pregnancy, the other third being caused by abruption and another third by 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 iceberg.
The exact reasons for implantation of the placenta in the lower uterine segment are not clearly understood. But the following theories have been postulated:
Falling Theory: It is believed that a poor decidual reaction in the upper part of the uterus can force the fertilized egg to fall and implant in the lower part of the uterus.
· Minor degrees of this condition can be explained by the persistence of chorionic activity in the decidua capsularis, leading to the formation of a large capsular placenta. This large capsular placenta in contact with the decidua vera of the lower segment is implanted in it.
· Defective decidua may result in compensatory proliferation of the placenta. This leads to a large placenta that can invade the lower segment.
A very large placenta (as in a twin pregnancy) can invade the lower segment of the uterus.
Some of the conditions that predispose to placenta previa are the following:
· Multiparity: This is a very important predisposing factor. Rates of this condition have been observed to increase with parity (parity refers to the number of times a woman has given birth).
Older maternal age: This is another important contributing factor. Women over the age of 35 have the highest chance of developing placenta previa.
Previous C-section: Previous C-section or any other surgery on the uterus that leaves a scar is known to increase the risk of placenta previa about twofold.
Placental size and abnormalities: As mentioned above, the size of the placenta affects the incidence of placenta previa. Apart from this, many placental abnormalities, such as succenturiate lobes, also increase the risk.