Placenta Previa (Low-Lying Placenta)

What is placenta previa (low-lying placenta)

Placenta, the flattish organ that develops during pregnancy to supply the baby with oxygen and nutrients as well as to get rid of the fetal wastes, is usually attached to the upper part of the uterine wall ^[1]. Sometimes, the placenta lies much lower in the uterus, blocking the cervix opening partially or even completely ^[2]. This condition is referred to as placental previa or low-lying placenta. Although most women with this problem give birth to healthy babies, in some cases, it can lead to a C-section or even premature delivery.

What causes placenta previa

Doctors are not yet sure about the exact causes responsible for the problem. In most cases, there is some uterine or placental irregularity that leads the placenta to remain attached to the lower parts of the uterus. ^[3].

Possible risk factors

Being over 35 years of age ^[4]
Having a history of C-section ^[5]
History of a low-lying placenta in a previous pregnancy
Having a history of multiple abortions ^[4]
Being pregnant for a second or subsequent time
Carrying twins or more (as it may require the placenta to grow larger, increasing its surface area) ^[6]
In vitro fertilization (IVF) ^[2]
Smoking or using cocaine ^[7]
Uterine surgeries and procedures such as dilation and curettage (D&C) and removal of uterine fibroids ^[8]

Having a placenta previa increases the chances of developing a placenta accreta, increta or percreta, where the placental tissues grow too deep into the uterine walls ^[9]. About 10% women with a previa develop this complication ^[10] with those having delivered a baby by a C-section in a previous pregnancy being more at risk ^[11].

Types of placenta previa

Partial placenta previa

As the name suggests, the placenta partially blocks the cervix, keeping the option for a natural vaginal delivery open ^[12].

Marginal placenta previa

The placenta grows in the lower part of the uterus, next to the cervical opening, threatening to block it as it grows in size. Vaginal delivery is possible in most cases, but there is a risk of bleeding if the edge of the placenta comes in the way during delivery ^[13].

Complete or total placenta previa

The major type, the placenta grows to completely cover the cervical opening ^[3].

In modern practice, these three types are sometimes merged into two basic categories: the marginal and complete placenta previa ^[4].

There is another, extremely rare type of previa where a small part of the placenta grows separately from the main placental body, at the edge of the cervix. It is connected to the main placenta by a blood vessel and can cover the cervical opening as it grows ^[14]. It is quite difficult to diagnose this type of previa in an early stage as it is often not easily detectable on ultrasounds.

What are the signs and symptoms of placenta previa

Painless bleeding, especially after the 20th-21st week is considered the most important characteristic symptoms, occurring in about 80% of all cases ^{[3, 15]}. Severity may range from light spotting to excessive bleeding that may go away on its own, only to return again after a few days ^[16]. Other symptoms may include:

Signs of early labor, such as abdominal cramps, lower back pain and contractions ^[17]
Bleeding after intercourses
Uterus measuring larger than its gestational age ^[18]
Baby remaining in a breech or transverse position due to lack of space in the lower uterus because of the placenta ^[19]

However, it is also possible to have no symptoms at all, with the problem being diagnosed in a routine ultrasound checkup ^[20].

When to call the doctor

Make sure to call the doctor if you experience heavy vaginal bleeding in the first trimester or any kind of bleeding during the second and third trimesters ^[16]. Also, watch out for signs of preterm labor such as pressure in the pelvic region and contractions increasing in frequency over time.

How is placenta previa diagnosed

Ultrasound

A transvaginal ultrasound is the most commonly used, safe procedure for examining the lower uterine region to check for an abnormally located placenta ^[21]. Abdominal ultrasound scans may also come useful to see if the placenta is blocking the uterus from opening into the cervix. A Doppler ultrasound may be used ^[22] in cases where placenta accreta is suspected ^[23].

MRI (Magnetic Resonance Imaging)

An MRI enables your doctor to examine the pelvic area to find out the exact location of the placenta. It also helps to confirm the diagnosis in case of a suspected placenta accreta ^[24].

Placenta previa treatment

The treatment is decided based on what stage of pregnancy you are in. Ninety percent of the times, placental previa detected before 20 weeks of gestation resolves without any treatment as the placenta gradually moves upward in the uterus, leaving the cervix open for a safe, natural birth ^[1]. However, if it stays attached to the lower uterine walls even after you enter your third trimester, the problem is likely to persist to interfere with labor and birth ^[25].

Management and delivery

There is no way to move the placenta, so the principal object of management is to continue the pregnancy for as long as possible, so the baby has a better chance of survival in case of a preterm delivery ^[26]. Regular ultrasound exams are performed to monitor the exact location of the placenta ^[27] as well as to assess the chances of any complication.

Women having a marginal placenta previa may have normal labor and delivery, but around 75% of women having the complication in the third trimester need a C-section ^[19].

Treatment of severe cases

Severe bleeding due to a previa may require hospitalization and blood transfusions ^[28] along with close monitoring to check for any risks to the baby. Your doctor may also prescribe corticosteroids to help your baby’s organs, especially the lungs, mature faster ^[29]. An amniocentesis may be done during the 36th–37th week to assess the fetal lung development before scheduling a cesarean section ^[24]. However, excessive bleeding around weeks 34 to 36 may call for an immediate C-section delivery ^[26]. Women with placenta accreta may need a hysterectomy due to excessive blood loss following delivery ^[22].

Placenta previa statistics: How common is it

About 6% of all pregnant women experience a low-lying placenta before the 20th week of gestation ^[3], with around 10% of these cases remaining into the third trimester. In 1 out of 200 pregnancies, the placenta stays in the lower uterus, blocking or threatening to block the cervical opening as the due date approaches ^[1].

Chances of recurrence

Women who have had a placenta previa in a previous pregnancy have 2%-3% chances of having it again ^[26].

Placenta previa vs. placental abruption (abruptio placentae)

Placental abruption occurs when the placenta starts to detach from its implantation site within the uterus, leading to bleeding and other serious complications ^[27]. It may occur along with a placenta previa or may be mistaken for the latter. The diagnosis is confirmed using an ultrasound scan.

ICD-9 and ICD-10 Codes

The ICD-9 codes 641.0 ^[30] and 641.1 ^[31] are used for referring to placenta previa while its ICD-10 codes are O44 ^[32] and P02.0 ^[33]

References +