Alcohol use during pregnancy was previously listed as a risk factor, but is discredited by this article. Classification
Traditionally, four grades of placenta previa were used, but it is now more common to simply differentiate between "major" and "minor cases. Other than that placenta previa can be also classified as:
Complete: When the placenta completely covers the cervix
Partial: When the placenta partially covers the cervix
Marginal: When the placenta ends near the edge of the cervix, about 2 cm from the internal cervical os

Diagnosis
History may reveal antepartum hemorrhage. Abdominal examination usually finds the uterus non-tender, soft and relaxed. Leopolds maneuvers may find the fetus in an oblique or breech position or lying transverse as a result of the abnormal position of the placenta. Malpresentation is found in about 35% cases. Vaginal examination is avoided in known cases of placenta previa. Confirmatory
Previa can be confirmed with an ultrasound. Transvaginal ultrasound has superior accuracy as compared to transabdominal one, thus allowing measurement of distance between placenta and cervical os. This has rendered traditional classification of placenta previa obsolete.False positives may be due to following reasons:
Overfilled bladder compressing lower uterine segment
Myometrial contraction simulating placental tissue in abnormally low location
Early pregnancy low position, which in third trimester may be entirely normal due to differential growth of the uterus.In such cases, repeat scanning is done after an interval of 15–30 minutes. 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 babys chance of survival is as good as at full term. Management
An initial assessment to determine the status of the mother and fetus is required. Although mothers used to be treated in the hospital from the first bleeding episode until birth, it is now considered safe to treat placenta previa on an outpatient basis if the fetus is at less than 30 weeks of gestation, and neither the mother nor the fetus are in distress. 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. Corticosteroids are indicated at 24–34 weeks gestation, given the higher risk of premature birth. Delivery
The method of delivery is determined by clinical state of the mother, fetus and ultrasound findings. In minor degrees (traditional grade I and II), vaginal delivery is possible. RCOG recommends that the placenta should be at least 2 cm away from internal os for an attempted vaginal delivery. When a vaginal delivery is attempted, consultant obstetrician and anesthetists are present in delivery suite. In cases of fetal distress and major degrees (traditional grade III and IV) a caesarean section is indicated. Caesarian section is contraindicated in cases of disseminated intravascular coagulation.