Placenta previa is a hazardous condition that occurs very rarely in pregnancy. It's when the placenta (attaching the baby to the lining of the uterus inside the mother) is directly over the cervix (the top of the vagina). There are several problems with placenta previa. The baby can't exit the uterus before the placenta. Since the placenta is the baby's life support system, the baby is unlikely to get out alive at all (through the vagina, anyway). Even if the baby, managed to be born alive this way, the blood loss for the mother will be considerable. Most women with placenta previa tend to have some if not a lot of bleeding throughout their pregnancy.

There are different grading systems for placenta previa, depending how close it is to the opening. If the placent is partly or entirely over the top of the vagina, then the baby will have to be born by cesarian section for the safety of mother and baby. If the placenta is within 2 inches of the vaginal opening, then a vaginal birth should be possible, but labour will need to be monitored closely.

 The placental attachment is in the lower segment of the uterine cavity. The Lower segment of the uterus is formed by isthmus and cervical canal. Clinically it is about 3 inches from the external os at full term gestation.

Clinically there are two types of placenta previa. The Major in which vaginal delivery is not possible, and Minor in which a vaginal delivery is possible.

Theoretically there are four types of placenta previa. They are as follows:

1. Lateral placenta previa
The placenta is only partly attached to the lower uterine segment; its lower margin dips into the lower segment but it cannot be easily reached by the examining finger through the non – dilated internal os.
2. Marginal placenta previa
A greater part of the placenta is attached to the lower segment of the uterine
Segment so that its lower margin reaches down to the internal os.
3. Partial placenta previa
The placenta completely over lies the non – dilated internal os, but the margin is still within reach of the examining finger.
4. Central placenta previa
Placenta over lies the non – dilated internal os and the margin of the placenta is too far from it to be reached by the examining finger.