Placenta previa: definition, causes, risk factors, symptoms, classification

By Cristiano Antonino On Dec 29, 2022

Placenta previa (or ‘placenta praevia’) is one of the obstetrical emergencies of the third trimester of pregnancy, caused by the fact that anatomically the placenta is located near or above the cervical opening, in front of the fetal presentation part (head, shoulders, podice)

Placenta previa is also often referred to as ‘low placenta’: the two terms are synonymous

They actually indicate the exact same thing.

Placenta previa decreases survival for the foetus and, in severe and untreated cases, can be fatal for both woman and foetus.

Vasa previa and placenta previa

Vasa previa (or ‘vasa previa’ or ‘vasa previ’) could be considered a type of placenta previa, however the two conditions are distinct.

Simplifying the concepts:

in vasa previa the blood vessels carrying nourishment to the foetus are located in front of or near the cervix;
on the contrary in placenta previa (or ‘low placenta’) it is the placenta itself that is placed in front of or near the cervix.

In both cases, the risks are high for both the woman and the foetus.

History

The condition was first described in 1685 by French physician Paul Portal (1630-1703).

Rates of the disease increased in the late 20th and early 21st century.

Epidemiology

Placenta previa affects about 0.5% of pregnancies, so it occurs in about one in every 200 births worldwide, but varies widely by region.

After four caesarean sections, however, it affects 10% of pregnancies, underlining the importance of one or more previous caesarean sections among the risk factors for placenta previa.

It has been suggested that rates of placenta previa are increasing due to the rise in caesarean section rates.

Reasons for regional variation may include ethnicity and diet.

Spread of placenta previa in Africa

Placenta previa rates in sub-Saharan Africa are the lowest in the world, averaging 2.7 per 1000 pregnancies.

Despite a low prevalence, this disease has had a profound impact in Africa as it is linked to adverse outcomes for both mother and child.

The most common maternal outcome of placenta previa is extreme blood loss before or after birth (antepartum haemorrhage and postpartum haemorrhage), which is a major cause of maternal and infant mortality in many African countries, such as Tanzania.

Risk factors for placenta previa among African women include previous pregnancies, prenatal alcohol consumption and inadequate gynaecological care.

In North Africa, placenta previa rates occur in 6.4 per 1000 pregnancies.

Prevalence in Asia

Mainland China has the highest prevalence of placenta previa in the world, with an average of 12.2 per 1000 pregnancies.

In particular, placenta previa is more common in south-east Asia, although the reason for this has not yet been studied.

There are many risk factors for placenta previa in Asian women, including pregnancies occurring in women aged 35 years or older (advanced maternal age) or in women who have had a previous caesarean section, who have had multiple pregnancies and who have suffered miscarriage or abortion in the past.

Compared to other Asian countries, placenta previa is more common in Japan (13.9 per 1000) and Korea (15 per 1000).

In the Middle East, placenta previa rates are lower in both Saudi Arabia (7.3 per 1000) and Israel (4.2 per 1000).

The continent with the second highest rate of placenta previa after Asia is Australia

It affects about 9.5 out of 1000 pregnant women.

Researchers interested in these rates tested the specificity and sensitivity of fetal abnormality scans.

It was determined that the threshold defining placenta previa (based on the proximity of the placenta to the cervix) should be reduced in order to improve the accuracy of diagnosis and avoid false positives leading to screening.

Prevalence in Europe and Italy

Placenta previa in Europe and Italy occurs in about 3.6 per 1000 pregnancies.

Diffusion in Central/South America

In Central and South America, placenta previa occurs in about 5.1 per 1000 pregnancies.

Diffusion in North America

In North America, placenta previa occurs in 2.9 per 1000 pregnancies.

Ethnic differences indicate that white women are less likely to experience placenta previa than black women.

In addition, more cases of placenta previa are found in women from low-income areas that are linked to inadequate pregnancy care.

According to the socioeconomic demographics of North America, black women are more likely to be from low-income areas and therefore more likely to develop placenta previa.

The incidence of placenta previa in the United States is increasing probably due to the increased rate of caesarean sections.

Signs and symptoms of placenta previa

The main signs and symptoms include vaginal bleeding that occurs in the second half of pregnancy.

The bleeding is bright red in colour and tends not to be associated with pain.

This commonly occurs around the 32nd week of gestation, but can occur even earlier.

More than half of women with placenta previa (51.6%) have bleeding before delivery.

This bleeding often starts mildly and may increase as the area of placental separation increases.

Placenta previa must be suspected if bleeding occurs after 24 weeks’ gestation.

Bleeding after delivery occurs in approximately 22% of those affected.

Women may also present as a case of failure of the fetal head to engage.

Placenta previa is basically an abnormal insertion of the placenta into the inner surface of the uterus.

Imagine the uterus as a bottle eaves placed upside down, where the thickest part is the body of the uterus and the neck of the bottle corresponds to the cervix.

The placenta can normally settle on the whole surface of the ‘bottle’ (such as on one of the sides or in the bottom, which is placed at the top), but – in the case of placenta previa – it settles on the neck of the ‘bottle’, the area which, as the months go by, will undergo contractions to prepare for labour and through which the baby will pass at the moment of birth.

Most of the time, as the uterus grows, the placenta moves upwards and everything resolves itself; in placenta previa, on the other hand, the placenta remains low until the end of gestation.

In short, placenta previa occurs when, immediately after conception, the embryo nestles in the mother’s uterus at a point in its lower segment: a fact that cannot be foreseen beforehand.

The exact cause of placenta previa is unknown

It is assumed to be related to abnormal vascularisation of the endometrium caused by scarring or atrophy from previous trauma, surgery or recurrent infections.

These factors may reduce the differential growth of the inferior segment, resulting in less upward displacement of the placental position as pregnancy progresses.

The following have been identified as risk factors for placenta previa

previous placenta previa (recurrence rate 4-8%)
one or more caesarean sections
myomectomy;
endometrial damage caused by curettage;
previous abortion;
twin birth;
large foetus;
inadequate gynaecological care;
low socio-economic status;
velamentous insertion of the umbilical cord;
various placental pathologies (succenturial lobes, bipartite placenta i.e. bilobed…);
baby in an unusual position: buttocks first or transverse (lying horizontally on the uterus). Incorrect presentation is found in about 35% of cases;
trauma, infection or surgery involving the uterus.

Women under the age of 20 are most at risk and women over the age of 35 are at increased risk as they get older.

Women who have had previous pregnancies (multiparity), particularly a large number of close pregnancies, are at increased risk due to uterine damage.

Smoking during pregnancy and cocaine use during pregnancy are undoubtedly predisposing factors.

Women with a large placenta from twins or erythroblastosis are at increased risk.

Ethnicity is a controversial risk factor, with some studies finding that people from Asia and African-Americans are at higher risk but others finding no difference.

Placenta previa is itself a risk factor for placenta accreta.

Alcohol consumption during pregnancy was previously listed as a risk factor, but this risk factor has since been removed (however, the fact remains that alcohol is not recommended for pregnant women).

Classification

Traditionally, four grades of placenta previa were used, but now it is more common to simply differentiate between ‘major cases’ and ‘minor cases’:

placenta minor: is located in the lower uterine segment, but the lower edge does not cover the internal operative system;
placenta major: is located in the lower uterine segment and the lower edge covers the internal os.

Placenta previa can also be classified as:

lateral placenta: when the placenta ends lateral to the edge of the cervix;
marginal placenta: when the placenta ends close to the edge of the cervix, about 2 cm from the inner cervical os
central placenta: when the placenta terminates at the cervix, which in turn is divided into
complete central placenta: when the placenta completely covers the cervix
partial central placenta: when the placenta partially covers the cervix

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