Cause of implantation of placenta in lower uterine segment

The exact cause of implantation of the placenta in the lower segment is not known.
The following theories are postulated :

1. Dropping down theory - The fertilized ovum drops down and is implanted in the
lower segment. Poor decidual reaction in the upper uterine segment may be the cause.
This explains the formation of central placenta praevia.

2. Defective decidua, resulting in spreading of the chorionic villi over a wide area in
the uterine wall to get nourishment. During this process, not only the placenta becomes
membranous but encroaches onto the lower uterine segment.

3. The formulation of capsular placenta - For chorion coming in contact with decidua
vera of the lower segment.

4. Big surface area of the placenta as in twins may encroach onto the lower segment.

The predisposing factors for placenta praevia -
- Multiparity.
- Increased maternal age (more than 35 years).
- History of previous caesarean section.
- Placental size.
- Smoking causes placental hypertrophy.