In the last trimester of pregnancy the isthmus of the uterus unfolds and forms the lower segment. In a normal pregnancy the placenta does not overlie it, so there is no bleeding. If the placenta does overlie the lower segment, it may shear off and a small section may bleed.
Women with placenta praevia often present with painless, bright red vaginal bleeding. This bleeding often starts mildly and may increase as the area of placental separation increases. Praevia should be suspected if there is bleeding after 24 weeks of gestation. Abdominal examination usually finds the uterus non-tender and relaxed. Leopold's Maneuvers may find the fetus in an oblique or breech position or lying transverse as a result of the abnormal position of the placenta. Praevia can be confirmed with an ultrasound. In parts of the world where ultrasound is unavailable, it is not uncommon to confirm the diagnosis with an examination in the surgical theatre.
The proper timing of an examination in theatre is important. If the woman is not bleeding severely she can be managed non-operatively until the 36th week. By this time the baby's chance of survival is as good as at full term.
Placenta praevia is classified according to the placement of the placenta:
Placenta praevia is itself a risk factor of placenta accreta.
Immediate delivery of the fetus may be indicated if the fetus is mature or if the fetus or mother are in distress. Blood volume replacement (to maintain blood pressure) and blood plasma replacement (to maintain fibrinogen levels) may be necessary.
It is controversial if vaginal delivery or a Ceasarean section is the safest method of delivery. In cases of fetal distress a Ceasarean section is indicated. Caesarian section is contraindicated in cases of disseminated intravascular coagulation.
A problem exists in places where a Ceasarean section cannot be performed, due to the lack of a surgeon or equipment. In these cases the infant can be delivered vaginally. There are two ways of doing this with a placenta praevia:
The goal of this type of delivery is to save the mother, and both methods will often kill the baby. These methods were used for many years before Ceasarean section and saved the lives of both mothers and babies with this condition.
The main risk with a vaginal delivery with a praevia is that as you are trying to bring down the head or a leg, you might separate more of the placenta and increase the bleeding.
Placenta praevia increases the risk of puerperal sepsis and postpartum haemorrhage because the lower segment to which the placenta was attached contracts less well post-delivery.