The following have been identified as risk factors for placenta previa:

- Previous placenta previa (recurrence rate 4–8%), caesarean delivery, myomectomy or endometrium damage caused by D&C.

- Women who are younger than 20 are at higher risk and women older than 35 are at increasing risk as they get older.

- Alcohol use during pregnancy was previous listed as a risk factor, but is discredited by this article.

- Women who have had previous pregnancies ( multiparity ), especially a large number of closely spaced pregnancies, are at higher risk due to uterine damage.

- Smoking during pregnancy; cocaine use during pregnancy

- Women with a large placentae from twins or erythroblastosis are at higher risk.

- Race is a controversial risk factor, with some studies finding that people from Asia and Africa are at higher risk and others finding no difference.

- Placental pathology (Vellamentous insertion, succinturiate lobes, bipartite i.e. bilobed placenta etc.)

- Baby is in an unusual position: breech (buttocks first) or transverse (lying horizontally across the womb).

Placenta previa is itself a risk factor of placenta accreta.

Placenta previa occurs approximately one of every 200 births. It has been suggested that incidence of placenta previa is increasing due to increased rate of Caesarian section.

Perinatal mortality rate of placenta previa is 3-4 times higher than normal pregnancies.

Factors increasing the risk (to either the woman, the fetus/es, or both) of pregnancy complications beyond the normal level of risk may be present in a woman's medical profile either before she becomes pregnant or during the pregnancy. These pre-existing factors may relate to physical and/or mental health, and/or to social issues, or a combination.

Some common risk factors include:

- Age of either parent

- Adolescent parents

- Older parents

- Exposure to environmental toxins in pregnancy

- Exposure to recreational drugs in pregnancy:

- Ethanol during pregnancy can cause fetal alcohol syndrome and fetal alcohol spectrum disorder.

- Tobacco smoking and pregnancy, when combined, causes twice the risk of premature rupture of membranes, placental abruption and placenta previa. Also, it causes 30% higher odds of the baby being born prematurely.

- Prenatal cocaine exposure is associated with, for example, premature birth, birth defects and attention deficit disorder.

- Prenatal methamphetamine exposure can cause premature birth and congenital abnormalities. Other investigations have revealed short-term neonatal outcomes to include small deficits in infant neurobehavioral function and growth restriction when compared to control infants. Also, prenatal methamphetamine use is believed to have long-term effects in terms of brain development, which may last for many years.

- Cannabis in pregnancy is possibly associated with adverse effects on the child later in life.

- Exposure to Pharmaceutical drugs in pregnancy. Anti-depressants, for example, may increase risks of such outcomes as preterm delivery.

- Ionizing radiation

- Risks arising from previous pregnancies:

- Complications experienced during a previous pregnancy are more likely to recur.

- Many previous pregnancies. Women who have had five previous pregnancies face increased risks of very rapid labor and excessive bleeding after delivery.

- Multiple previous fetuses. Women who have had more than one fetus in a previous pregnancy face increased risk of mislocated placenta.

- Multiple pregnancy, that is, having more than one fetus in a single pregnancy.

- Social and socioeconomic factors. Generally speaking, unmarried women and those in lower socioeconomic groups experience an increased level of risk in pregnancy, due at least in part to lack of access to appropriate prenatal care.

- Unintended pregnancy. Unintended pregnancies preclude preconception care and delays prenatal care. They preclude other preventive care, may disrupt life plans and on average have worse health and psychological outcomes for the mother and, if birth occurs, the child.

- Height. Pregnancy in women whose height is less than 1.5 meters (5 feet) correlates with higher incidences of preterm birth and underweight babies. Also, these women are more likely to have a small pelvis, which can result in such complications during childbirth as shoulder dystocia.

- Weight

- Low weight: Women whose pre-pregnancy weight is less than 45.5 kilograms (100 pounds) are more likely to have underweight babies.

- Obese women are more likely to have very large babies, potentially increasing difficulties in childbirth. Obesity also increases the chances of developing gestational diabetes, high blood pressure, preeclampsia, experiencing postterm pregnancy and/or requiring a cesarean delivery.

- Intercurrent disease in pregnancy, that is, a disease and condition not necessarily directly caused by the pregnancy, such as diabetes mellitus in pregnancy, SLE in pregnancy or thyroid disease in pregnancy.

It is recommended that women with vasa previa should deliver through elective cesarean prior to rupture of the membranes. Given the timing of membrane rupture is difficult to predict, elective cesarean delivery at 35–36 weeks is recommended. This gestational age gives a reasonable balance between the risk of death and that of prematurity. Several authorities have recommended hospital admission about 32 weeks. This is to give the patient proximity to the operating room for emergency delivery should the membranes rupture. Because these patients are at risk for preterm delivery, it is recommended that steroids should be given to promote fetal lung maturation. When bleeding occurs, the patient goes into labor, or if the membranes rupture, immediate treatment with an emergency caesarean delivery is usually indicated.

In rare cases, inherited bleeding disorders, like hemophilia, von Willebrand disease (vWD), or factor IX or XI deficiency, may cause severe postpartum hemorrhage, with an increased risk of death particularly in the postpartum period. The risk of postpartum hemorrhage in patients with vWD and carriers of hemophilia has been found to be 18.5% and 22% respectively. This pathology occurs due to the normal physiological drop in maternal clotting factors after delivery which greatly increases the risk of secondary postpartum hemorrhage.

Another bleeding risk factor is thrombocytopenia, or decreased platelet levels, which is the most common hematological change associated with pregnancy induced hypertension. If platelet counts drop less than 100,000 per microliter the patient will be at a severe risk for inability to clot during and after delivery.

Vasa previa is seen more commonly with velamentous insertion of the umbilical cord, accessory placental lobes (succenturiate or bilobate placenta), multiple gestation, IVF pregnancy. In IVF pregnancies incidences as high as one in 300 have been reported. The reasons for this association are not clear, but disturbed orientation of the blastocyst at implantation, vanishing embryos and the increased frequency of placental morphological variations in in vitro fertilisation pregnancies have all been postulated.

Some disorders and conditions can mean that pregnancy is considered high-risk (about 6-8% of pregnancies in the USA) and in extreme cases may be contraindicated. High-risk pregnancies are the main focus of doctors specialising in maternal-fetal medicine.

Serious pre-existing disorders which can reduce a woman's physical ability to survive pregnancy include a range of congenital defects (that is, conditions with which the woman herself was born, for example, those of the heart or , some of which are listed above) and diseases acquired at any time during the woman's life.

Antepartum bleeding (APH), also prepartum hemorrhage, is bleeding during pregnancy from the 24th week (sometimes defined as from the 20th week) gestational age to full term (40th week). The primary consideration is the presence of a placenta previa which is a low lying placenta at or very near to the internal cervical os. This condition occurs in roughly 4 out of 1000 pregnancies and usually needs to be resolved by delivering the baby via cesarean section. Also a placental abruption (in which there is premature separation of the placenta) can lead to obstetrical hemorrhage, sometimes concealed. This pathology is of important consideration after maternal trauma such as a motor vehicle accident or fall.

Other considerations to include when assessing antepartum bleeding are: sterile vaginal exams that are performed in order to assess dilation of the patient when the 40th week is approaching. As well as cervical insufficiency defined as a midtrimester (14th-26th week) dilation of the cervix which may need medical intervention to assist in keeping the pregnancy sustainable.

An important risk factor for placenta accreta is placenta previa in the presence of a uterine scar. Placenta previa is an independent risk factor for placenta accreta. Additional reported risk factors for placenta accreta include maternal age and multiparity, other prior uterine surgery, prior uterine curettage, uterine irradiation, endometrial ablation, Asherman syndrome, uterine leiomyomata, uterine anomalies, hypertensive disorders of pregnancy, and smoking.

The condition is increased in incidence by the presence of scar tissue i.e. Asherman's syndrome usually from past uterine surgery, especially from a past dilation and curettage, (which is used for many indications including miscarriage, termination, and postpartum hemorrhage), myomectomy, or caesarean section. A thin decidua can also be a contributing factor to such trophoblastic invasion. Some studies suggest that the rate of incidence is higher when the fetus is female. Other risk factors include low-lying placenta, anterior placenta, congenital or acquired uterine defects (such as uterine septa), leiomyoma, ectopic implantation of placenta (including cornual pregnancy).

Pregnant women above 35 years of age who have had a Caesarian section and now have a placenta previa overlying the uterine scar have a 40% chance of placenta accreta.

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. Women who have high blood pressure or who have had a previous placental abruption and want to conceive must be closely supervised by a doctor.

The risk of placental abruption can be reduced by maintaining a good diet including taking folic acid, regular sleep patterns and correction of pregnancy-induced hypertension.

It is crucial for women to be made aware of the signs of placental abruption, such as vaginal bleeding, and that if they experience such symptoms they must get into contact with their health care provider/the hospital "without any delay".

The prognosis of this complication depends on whether treatment is received by the patient, on the quality of treatment, and on the severity of the abruption. Outcomes for the baby also depend on the gestational age.

In the Western world, maternal deaths due to placental abruption are rare. The fetal prognosis is worse than the maternal prognosis; approximately 12% of fetuses affected by placental abruption die. 77% of fetuses that die from placental abruption die before birth; the remainder die due to complications of preterm birth.

Without any form of medical intervention, as often happens in many parts of the world, placental abruption has a high maternal mortality rate.

Because pregnancy is outside the uterus, abdominal pregnancy serves as a model of human male pregnancy or for females who lack a uterus, although such pregnancy would be dangerous.

Cases of combined simultaneous abdominal and intrauterine pregnancy have been reported.

Risk factors are similar to tubal pregnancy with sexually transmitted disease playing a major role; however about half of those with ectopic pregnancy have no known risk factors (which include damage to the Fallopian tubes from previous surgery or from previous ectopic pregnancy, and tobacco smoking).

The reported incidence of placenta accreta has increased from approximately 0.8 per 1000 deliveries in the 1980s to 3 per 1000 deliveries in the past decade.

Incidence has been increasing with increased rates of Caesarean deliveries, with rates of 1 in 4,027 pregnancies in the 1970s, 1 in 2,510 in the 1980s, and 1 in 533 for 1982–2002. In 2002, ACOG estimated that incidence has increased 10-fold over the past 50 years. The risk of placenta accreta in future deliveries after Caesarian section is 0.4-0.8%. For patients with placenta previa, risk increases with number of previous Caesarean sections, with rates of 3%, 11%, 40%, 61%, and 67% for the first, second, third, fourth, and fifth or greater number of Caesarean sections.

In humans, retained placenta is generally defined as a placenta that has not undergone placental expulsion within 30 minutes of the baby’s birth where the third stage of labor has been managed actively.

Risks of retained placenta include hemorrhage and infection. After the placenta is delivered, the uterus should contract down to close off all the blood vessels inside the uterus. If the placenta only partially separates, the uterus cannot contract properly, so the blood vessels inside will continue to bleed. A retained placenta thereby leads to hemorrhage.

Many factors can contribute to the loss of uterine muscle tone, including:

- overdistention of the uterus

- multiple gestations

- polyhydramnios

- fetal macrosomia

- prolonged labor

- oxytocin augmentation of labor

- grand multiparity (having given birth 5 or more times)

- precipitous labor (labor lasting less than 3 hours)

- magnesium sulfate treatment of preeclampsia

- chorioamnionitis

- halogenated anesthetics

- uterine leiomyomata

- full bladder

- retained colyledon, placental fragments

- placenta previa

- placental abruption

- constriction ring

- incomplete separation of the placenta

True cervical pregnancies tend to abort; if, however, the pregnancy is located higher in the canal and the placenta finds support in the uterine cavity it can go past the first trimester. With the placenta being implanted abnormally extensive vaginal bleeding can be expected at time of delivery and placental removal. While early cervical pregnancies may abort spontaneously or can be managed with excision, D&C, suturing, electrocautery, and tamponading, by medication such as methotrexate, and/or by uterine artery embolization, a more advanced pregnancy may require a hysterectomy to control bleeding. The more advanced the pregnancy the higher the risk for a major bleeding necessitating a hysterectomy.

On a very rare occasion, a cervical pregnancy results in the birth of a live baby, typically the pregnancy is in the upper part of the cervical canal and manages to extend into the lower part of the uterine cavity.

A cervical pregnancy can develop together with a normal intrauterine pregnancy; such a heterotopic pregnancy will call for expert management as to not to endanger the intrauterine pregnancy.

There is no effective pharmacological treatment for retained placenta. It is useful ensuring the bladder is empty. However, ergometrine should not be given as it causes tonic uterine contractions which may delay placental expulsion. Controlled cord traction has been recommended as a second alternative after more than 30 minutes have passed after stimulation of uterine contractions, provided the uterus is contracted. Manual extraction may be required if cord traction also fails, or if heavy ongoing bleeding occurs. Very rarely a curettage is necessary to ensure that no remnants of the placenta remain (in rare conditions with very adherent placenta such as a placenta accreta).

However, in birth centers and attended home birth environments, it is common for licensed care providers to wait for the placenta's birth up to 2 hours in some instances.

Circumvallate placenta is a placental morphological abnormalitiy, a subtype of placenta extrachorialis in which the fetal membranes (chorion and amnion) "double back" on the fetal side around the edge of the placenta. After delivery, a circumvallate placenta has a thick ring of membranes on its fetal surface.

The fetal surface is divided into a central depressed zone surrounded by a thickened white ring which is incomplete the ring is situated at varying distance from the margin of the placenta. The ring is composed of a double fold of amnion and chorion with degenerated decidua vera and fibrin in between. Vessels radiate from the cord insertion as far as the ring and then disappear from the view.

Complete circumvallate placenta occurs in approximately 1% of pregnancies. It is diagnosed prenatally by medical ultrasonography, although one 1997 study of prenatal ultrasounds found that "of the normal placentas, 35% were graded as probably or definitely circumvallate by at least one sonologist," and "all sonologists misgraded the case of complete circumvallation as normal." The condition is associated with perinatal complications such as placental abruption, oligohydramnios, abnormal cardiotocography, preterm birth, and miscarriage.

AS has a reported incidence of 25% of D&Cs performed 1–4 weeks post-partum, up to 30.9% of D&Cs performed for missed miscarriages and 6.4% of D&Cs performed for incomplete miscarriages. In another study, 40% of patients who underwent repeated D&C for retained products of conception after missed miscarriage or retained placenta developed AS.

In the case of missed miscarriages, the time period between fetal demise and curettage may increase the likelihood of adhesion formation due to fibroblastic activity of the remaining tissue.

The risk of AS also increases with the number of procedures: one study estimated the risk to be 16% after one D&C and 32% after 3 or more D&Cs. However, a single curettage often underlies the condition.

In an attempts to estimate the prevalence of AS in the general population, it was found in 1.5% of women undergoing hysterosalpingography HSG, and between 5 and 39% of women with recurrent miscarriage.

After miscarriage, a review estimated the prevalence of AS to be approximately 20% (95% confidence interval: 13% to 28%).

A number of studies have shown that tobacco use is a significant factor in miscarriages among pregnant smokers, and that it contributes to a number of other threats to the health of the fetus. Smoking and pregnancy, combined, cause twice the risk of premature rupture of membranes, placental abruption and placenta previa. Also, it causes 30% higher odds of the baby being born prematurely.

The most common cause is the mismanagement of 3rd stage of labor, such as:

- Fundal pressure

- Excess cord traction during the 3rd stage of labor

Other natural causes can be:

- Uterine weakness, congenital or not

- Precipitate delivery

- Short umbilical cord

It is more common in multiple gestation than in singleton pregnancies.

The incidence is of 1/2000 pregnancies.

Methods of measuring blood loss associated with childbirth vary, complicating comparison of prevalence rates. A systematic review reported the highest rates of PPH in Africa (27.5%), and the lowest in Oceania (7.2%), with an overall rate globally of 10.8%. The rate in both Europe and North America was around 13%. The rate is higher for multiple pregnancies (32.4% compared with 10.6% for singletons), and for first-time mothers (12.9% compared with 10.0% for women in subsequent pregnancies). The overall rate of severe PPH (>1000 ml) was much lower at an overall rate of 2.8%, again with the highest rate in Africa (5.1%).

Velamentous cord insertion is an abnormal condition during pregnancy. Normally, the umbilical cord inserts into the middle of the placenta as it develops. In velamentous cord insertion, the umbilical cord inserts into the fetal membranes (choriamniotic membranes), then travels within the membranes to the placenta (between the amnion and the chorion). The exposed vessels are not protected by Wharton's jelly and hence are vulnerable to rupture. Rupture is especially likely if the vessels are near the cervix, in which case they may rupture in early labor, likely resulting in a stillbirth. This is a serious condition called vasa previa. Not every pregnancy with a velamentous cord insertion results in vasa previa, only those in which the blood vessels are near the cervix.

When a velamentous cord insertion is discovered, the obstetrician will monitor the pregnancy closely for the presence of vasa previa. If the blood vessels are near the cervix, the baby will be delivered via cesarean section as early as 35 weeks to prevent the mother from going into labor, which is associated with a high infant mortality. Early detection can reduce the need for emergency cesarean sections.

Uterine inversion is often associated with significant Post-partum hemorrhage. Traditionally it was thought that it presented with haemodynamic shock "out of proportion" with blood loss, however blood loss has often been underestimated. The parasympathetic effect of traction on the uterine ligaments may cause bradycardia.