Summary

• During pregnancy, the placenta provides the growing baby with oxygen and nutrients from the mother’s bloodstream.
• Placenta previa means the placenta has implanted at the bottom of the uterus, over the cervix or close by, which means the baby can’t be born vaginally.
• Treatment aims to ease the symptoms and prolong the pregnancy until at least 36 weeks.

During pregnancy, the placenta provides the growing baby with oxygen and nutrients from the mother’s bloodstream. Placenta previa means the placenta has implanted at the bottom of the uterus, covering the cervix.

When a baby is ready to be born, the cervix (neck of the womb) dilates (opens) to allow the baby to move out of the uterus and into the vagina. When a woman has placenta previa (the placenta has implanted at the bottom of the uterus, over the cervix or close by), the baby can’t be born vaginally. ‘Partial placenta previa’ means the cervix is partly blocked, while ‘complete placenta previa’ means the entire cervix is obstructed.

Some of the causes include scarring of the uterine lining (endometrium) and abnormalities of the placenta. Around one in every 200 pregnancies is affected.

The gestation period is divided into three equal periods or trimesters. During the first two to four weeks of the first trimester, nutrition and waste are handled by the endometrial lining through diffusion. As the trimester progresses, the outer layer of the embryo begins to merge with the endometrium, and the placenta forms. This organ takes over the nutrient and waste requirements of the embryo and fetus, with the parent’s blood passing nutrients to the placenta and removing waste from it. Chemicals from the fetus, such as bilirubin, are processed by the parent’s liver for elimination. Some of the parent’s immunoglobulins will pass through the placenta, providing passive immunity against some potential infections.

Internal organs and body structures begin to develop during the first trimester. By five weeks, limb buds, eyes, the heart, and liver have been basically formed. By eight weeks, the term fetus applies, and the body is essentially formed, as shown in Figure 43.19. The individual is about five centimeters (two inches) in length and many of the organs, such as the lungs and liver, are not yet functioning. Exposure to any toxins is especially dangerous during the first trimester, as all of the body’s organs and structures are going through initial development. Anything that affects that development can have a severe effect on the fetus’ survival.

During the second trimester, the fetus grows to about 30 cm (12 inches), as shown in Figure 43.20. It becomes active and the pregnant person usually feels the first movements. All organs and structures continue to develop. The placenta has taken over the functions of nutrition and waste and the production of estrogen and progesterone from the corpus luteum, which has degenerated. The placenta will continue functioning up through the delivery of the baby.

During the third trimester, the fetus grows to 3 to 4 kg (6 ½ -8 ½ lbs.) and about 50 cm (19-20 inches) long, as illustrated in Figure 43.21. This is the period of the most rapid growth during the pregnancy. Organ development continues to birth (and some systems, such as the nervous system and liver, continue to develop after birth). The pregnant person will be most uncomfortable during this trimester. They may urinate frequently due to pressure on the bladder from the fetus. There may also be intestinal blockage and circulatory problems, especially in the legs, where clots may form due to pressure from the fetus on returning veins as they enter the abdominal cavity.

Some of the possible causes and risk factors of placenta previa include:

• Low implantation of the fertilised egg
• Abnormalities of the uterine lining, such as fibroids
• Scarring of the uterine lining (endometrium)
• Abnormalities of the placenta
• Multiple babies, such as twins
• Multiple pregnancies - a woman who has already had six or more deliveries has a risk of one in 20.

The most important symptom in placenta previa is painless vaginal bleeding after 20 weeks. However, there are causes of vaginal bleeding other than placenta previa. All bleeding during pregnancy should be reported to your doctor for prompt investigation and treatment.

Why the bleeding happens

During the later stages of pregnancy, the bottom part of the uterus thins and spreads to accommodate the growing baby. If the placenta is anchored to the bottom of the uterus (as occurs with placenta previa), this thinning and spreading separates the placenta and causes bleeding.

Sexual intercourse can also cause bleeding from the placenta previa in later pregnancy. During labour, the cervix thins and dilates, which would normally allow the baby to exit into the vagina. In placenta previa, the dilation of the cervix further tears the placenta and causes bleeding.

A pregnant woman who experiences any vaginal bleeding should be admitted to hospital and tested. Some of the tests used to diagnose placenta previa include:

• Ultrasound scan
• Feeling the mother’s belly to establish the baby’s position (the baby is sideways or presenting bottom-first in around one in three cases of placenta previa).

Digital vaginal examinations should be strictly avoided

It is sometimes difficult to tell the difference between placenta previa and placental abruption. Placental abruption is a condition where the placenta separates from the uterine wall. Both conditions are flagged by heavy bleeding of bright red blood.

A vaginal examination is often used to help diagnose placental abruption, but could trigger heavier bleeding in the case of placenta previa. An ultrasound scan should always be taken first, and digital (finger) vaginal examinations strictly avoided in the case of placenta previa.

The doctor may do a speculum vaginal examination very gently to make sure the bleeding is not coming from the cervix or vagina. Once the diagnosis is made, the pregnancy needs to be very carefully monitored. Placenta previa is a potentially life-threatening condition for the both the mother and her baby.

Treatment options vary

Treatment depends on a number of factors, including:

• Whether the placenta previa is complete or partial
• The exact location of the placenta
• The amount of blood lost
• The gestational age of the baby
• The position of the baby
• The health of the baby
• The health of the mother.

Treatment during pregnancy

Medical treatment aims to ease the symptoms and prolong the pregnancy. Options may include:

• Bed rest.
• Hospitalisation.
• Close monitoring, such as using a fetal monitor and regularly checking the mother’s vital signs (for example, blood pressure).
• Blood transfusion for the mother.
• Avoiding any activity that triggers uterine contractions or irritates the cervix, such as sexual intercourse or orgasms.

Delivery

Once the baby is old enough to be delivered, a caesarean section is usually performed. The baby may need to be monitored in intensive care to make sure all is well. The mother will undergo a range of tests, including tests to check her blood cell counts and the clotting ability of her blood.

Some of the complications of placenta previa include:

• Major haemorrhage (bleeding) for the mother
• Shock from loss of blood
• Fetal distress from lack of oxygen
• Premature labour or delivery
• Health risks to the baby, if born prematurely
• Emergency caesarean delivery
• Hysterectomy, if the placenta fails to come away from the uterine lining
• Blood loss for the baby
• Death.