Common symptoms include:

sudden-onset abdominal pain
contractions that seem continuous and do not stop
vaginal bleeding
enlarged uterus (disproportionate to the gestational age of the fetus)
decreased fetal movement
decreased fetal heart rate.Vaginal bleeding, if it occurs, may be bright red or dark.A placental abruption caused by arterial bleeding at the center of the placenta leads to sudden development of severe symptoms and life-threatening conditions including fetal heart rate abnormalities, severe maternal hemorrhage, and disseminated intravascular coagulation (DIC). Those abruptions caused by venous bleeding at the periphery of the placenta develop more slowly and cause small amounts of bleeding, intrauterine growth restriction, and oligohydramnios (low levels of amniotic fluid). Risk factors
Pre-eclampsia
Chronic hypertension
Short umbilical cord
Premature rupture of membranes
Prolonged rupture of membranes (>24 hours). Thrombophilia
Polyhydramnios
Multiparity
Multiple pregnancy
Maternal age: pregnant women who are younger than 20 or older than 35 are at greater riskRisk factors for placental abruption include disease, trauma, history, anatomy, and exposure to substances. The risk of placental abruption increases sixfold after severe maternal trauma. Anatomical risk factors include uncommon uterine anatomy (e.g. bicornuate uterus), uterine synechiae, and leiomyoma. Substances that increase risk of placental abruption include cocaine and tobacco when consumed during pregnancy, especially the third trimester. History of placental abruption or previous Caesarian section increases the risk by a factor of 2.3. Pathophysiology
In the vast majority of cases, placental abruption is caused by the maternal vessels tearing away from the decidua basalis, not the fetal vessels. The underlying cause is often unknown. A small number of abruptions are caused by trauma that stretches the uterus.
The diagnosis is one of exclusion, meaning other possible sources of vaginal bleeding or abdominal pain have to be ruled out in order to diagnose placental abruption. Of note, use of magnetic resonance imaging has been found to be highly sensitive in depicting placental abruption, and may be considered if no ultrasound evidence of placental abruption is present, especially if the diagnosis of placental abruption would change management. Classification
Based on severity:

Class 0: Asymptomatic. Diagnosis is made retrospectively by finding an organized blood clot or a depressed area on a delivered placenta. Class 1: Mild and represents approximately 48% of all cases. Characteristics include the following:
No vaginal bleeding to mild vaginal bleeding
Slightly tender uterus
Normal maternal blood pressure and heart rate
No coagulopathy
No fetal distress
Class 2: Moderate and represents approximately 27% of all cases. Characteristics include the following:
No vaginal bleeding to moderate vaginal bleeding
Moderate-to-severe uterine tenderness with possible tetanic contractions
Maternal tachycardia with orthostatic changes in blood pressure and heart rate
Fetal distress
Hypofibrinogenemia (i.e., 50–250 mg/dL)
Class 3: Severe and represents approximately 24% of all cases. Characteristics include the following:
No vaginal bleeding to heavy vaginal bleeding
Very painful tetanic uterus
Maternal shock
Hypofibrinogenemia (i.e., <150 mg/dL)
Coagulopathy
Fetal death

Prevention
Although the risk of placental abruption cannot be eliminated, it can be reduced. Avoiding tobacco, alcohol and cocaine during pregnancy decreases the risk. Staying away from activities which have a high risk of physical trauma is also important.