Placenta Previa

Placenta previa is a potentially serious complication of pregnancy where the placenta implants into the lower segment of the uterus. This section outlines the complications associated with this condition.

Contents

What is placenta previa?
Causes
Diagnosis
Risk factors
Management
Role of placental function testing
Prognosis following a pregnancy with placenta previa

What is placenta previa?

The fertilized egg enters the upper part of the uterus from one of the fallopian tubes. It normally implants in the upper part of the uterus (the body). Its location within the body can be posterior (on the back wall), fundal (at the top of the uterus; as shown below for ‘normal placenta’), or anterior (on the front wall). Placenta previa is a potentially serious complication of pregnancy where the placenta implants into the lower segment of the uterus (defined as the portion of the uterus within the maternal pelvis). Placenta previa affects around 1 in 200 pregnancies and is increasing in incidence (see risk factors below).

Classification of Placenta Previa

There are 4 types of placenta previa as follows:

–        Low-lying placenta: the lower edge of the placenta is within the lower uterine segment;
–        Partial or marginal placenta previa: the lower edge of the placenta reaches to within 2cm of the internal os (internal cervical opening);
–        Placenta previa: the lower edge of the placenta crosses the internal os;
–        Major or complete placenta previa: the placenta has a central location within the lower segment and covers the cervix.

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Can placenta previa go away as pregnancy advances?

Yes, placenta previa can go away as the pregnancy progresses. The minor types of placenta previa that do not cross the internal os (internal opening) of the cervix will appear to migrate upwards as the pregnancy advances. The reason for this is that as pregnancy progresses, the lower uterine segment gradually thins out and the cervix shortens, in association with Braxton-Hicks contractions (these are ‘practice’ contractions) that gradually increase in the third trimester. These practice contractions are normal and serve to gradually prepare the uterus and cervix for normal labour at term. During this apparent upward migration, the placenta remains attached to its original location.

Potential complications arising from placenta previa include:

–        Caesarean delivery: due to vaginal bleeding, abnormal lie [transverse lie], or abnormal presentation [breech presentation] of the fetus;
–        Antepartum hemorrhage (APH): when the placenta crosses over the internal os, it is unable to ‘move’ with the uterine thinning process (the resultant stretching) in the upward direction, causing placental separation. Maternal blood is thus released through the cervix into the vagina. Classically, this bleeding is painless, in contrast to bleeding from a normally-located placenta (known as placental abruption);
–        Preterm delivery:  most episodes of vaginal bleeding are mild and stop by themselves, the bleeding is fresh, bright red, and causes alarm, thus prompting hospital assessment followed by admission until it has ceased. Occasionally, very heavy vaginal bleeding can occur that requires a blood transfusion. Unless such bleeding stops, delivery by Caesarean section is required in order to remove the placenta and stop the bleeding. Since the fetus may not be full term when this bleeding occurs, this may result in a preterm delivery;
–        Invasive placenta (see invasive placenta[k1]  for more information);
–        Accurate diagnosis and appropriate management results in favourable outcomes for the majority of women with this condition.

Causes of placenta previa

There is no single cause of this condition. It has no underlying genetic factors. A woman’s risk is determined by the presence or absence of risk factors described below.

Diagnosing placenta previa

In most cases, placenta previa is suspected at the time of the routine fetal anatomy exam at 18-20 weeks. This examination is an abdominal (belly) ultrasound that is usually capable of making an accurate diagnosis of major or complete placenta previa. Intermediate types (partial/marginal placenta previa, and placenta previa, as described above) usually require a transvaginal ultrasound examination.

Symptoms and Signs

Painless, bright red vaginal bleeding in the late second or third trimester is suggesting of placenta previa. The bleeding may range from light to heavy, and may be accompanied by uterine cramping, but is typically not associated with abdominal pain.

Tests

Ultrasound scan

–        Transabdominal ultrasound: ultrasound of the abdomen, this can determine the location of the placenta in the uterus
–        Transvaginal ultrasound (TVS; transvaginal sonography): this is the most accurate way to measure the distance from the internal opening of the cervix (the internal os) to the edge of the placenta

Risk Factors

The following factors can increase a woman’s risk of placenta previa:

–        Previous Caesarean section
–        Advanced maternal age
–        Multiparity (more than one previous pregnancy)
–        Multi-fetal pregnancy (more than one fetus in the current pregnancy)
–        Smoking
–        In-vitro fertilization (IVF)
–        Previous uterine surgery such as myomectomy, septoplasty, multiple D&Cs, adhesiolysis for Asherman syndrome (see risk factors for invasive placenta[k2]  for more information)

Management of placenta previa

How can I maximize the safety of myself and my baby?

The first step is to undertake appropriate ultrasound examinations to define the type of placenta previa. If placenta previa has occurred in the context of previous uterine surgery, especially with multiple Caesarean sections, special consideration needs to be given to the possibility of an invasive placenta.

Uncomplicated placenta previa crossing the internal os, but not causing any bleeding, does not require admission to hospital.

Women with minor degrees of placenta previa

Many instances of minor placenta previa can resolve during the third trimester, such that the lower edge of the placenta is at least 2 cm away from the internal os. One or more TVS examinations in the 32-37 week period may be needed to confirm that the placenta previa has resolved. Such examinations are worth pursuing even after 34 weeks in situations of minor placenta previa because it is the onset of Braxton-Hicks contractions that may provoke thinning of the lower uterine segment and the apparent placental migration as the cervix shortens.

Most instances of minor placenta previa cause none or minimal vaginal bleeding and, thus, rarely require admission to a hospital. If a TVS shows that the lower placenta edge has moved more than 2 cm from the internal os, then attempting vaginal delivery is safe.

Women with major placenta previa

Women at home with major placenta previa are advised to:

–        Take a daily iron supplement to maintain their hemoglobin levels
–        Avoid constipation (straining)
–        Avoid intercourse
–        Avoid highway driving (in case of sudden bleeding)
–        Remain within accessible driving distance or ambulance access to a major hospital
–        Avoid travel to remote areas, or air/boat travel

Up to 50% of women with major placenta previa will require admission to hospital due to bleeding before a planned delivery date by Caesarean section. The additional precautions for major placenta previa can include some or all of the following steps during their Caesarean section:

–        Blood cross match/transfusion
–        The procedure performed 1-2 weeks earlier than normal
–        A midline skin incision below the umbilicus (belly button)
–        Midline (or classical) Caesarean section
–        Additional skilled staff to assist

Maternal complications for a Caesarean section for major placenta previa include the following:

–        Need for general anaesthetic for the surgery
–        Risk of requiring interventional radiology techniques to arrest excessive bleeding during the operating room after the Caesarean section
–        Bladder injury at Caesarean section (during attempts to arrest bleeding)
–        Caesarean hysterectomy (rarely needed to stop internal bleeding)
–        Blood transfusion
–        Need for admission to intensive care due to excessive blood loss

Role of placental function testing in placenta previa

Placental function is usually normal in women with placenta previa. Therefore, hypertension in pregnancy (pre-eclampsia[k3] ) and/or poor growth of the baby (IUGR[k4] ) are very uncommon.

Prognosis following a pregnancy with placenta previa

Mother’s health

Women who have an uncomplicated Caesarean section for placenta previa should discuss their risk factors for recurrence at their 6 week post-partum visit

Baby’s health

The main threat to the developing baby is the need for early delivery by Caesarean section due to the occasionally very heavy nature of the vaginal bleeding that could threaten the safety of the mother if she does not have a Caesarean section at that time.

Future pregnancies

Women who have experienced placenta previa in pregnancy are advised to defer pregnancy for more than 1 year to minimize future risk. Patients contemplating future pregnancies following a Caesarean section for placenta previa need to appreciate the risk and consequences of recurrence with placental invasion.

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