A blighted ovum, also known as an anembryonic pregnancy or anembryonic gestation, is a type of pregnancy in which a viable embryo never develops after the egg has been fertilized and attaches itself to the wall of the uterus. A woman who experiences an anembryonic pregnancy may have normal pregnancy symptoms for a while, but a blighted ovum will not grow into a fetus.
Symptoms of a Blighted Ovum
A blighted ovum occurs very early in pregnancy, often before the woman even realizes she is pregnant. During the first month or so of anembryonic pregnancy, an ultrasound will most likely show that everything appears to be normal. The woman may experience such typical pregnancy symptoms as breast tenderness, morning sickness, and elevated levels of the hCG pregnancy hormone. In most cases she will miss one or two periods, and if a pregnancy test is administered it will show a positive result.
Some women will experience abdominal cramps, or vaginal spotting or bleeding, but in most cases the blighted ovum will not cause unusual symptoms and will not be detected until an empty gestational sac is observed in an ultrasound examination later in the pregnancy.
How it Happens
In a normal pregnancy, the fertilized egg becomes a collection of cells known as a blastocyst, which eventually develops into an embryo, a fetus, and finally a child who won’t eat his vegetables. The blastocyst is covered by a layer of cells called the trophoblast (the individual cells that make up the trophoblast are also called trophoblasts), which helps the blastocyst to attach itself to the uterine wall. The trophoblast layer eventually becomes the placenta, the organ responsible for providing the developing fetus with oxygen and nourishment. In an anembryonic pregnancy, however, the trophoblast attaches itself to the uterus, but the blastocyst never fully forms.
Interestingly, a recent study found significantly fewer trophoblasts in women who had been diagnosed with ectopic pregnancy (an embryo attached outside the uterus) or blighted ovum.
Causes of a Blighted Ovum
This condition is believed to be caused by severe chromosomal abnormalities in the fertilized egg, which naturally cause the woman’s body to miscarry (this may be an evolutionary adaptation that serves to prevent birth defects). These kinds of abnormalities are sometimes caused by poor sperm quality, or by imperfections in the egg (which become more likely after the woman is over the age of 35), although the chromosomes of both parents are usually normal. Blighted ova are the cause of nearly half of all miscarriages that occur during the first trimester.
Diagnosis of a Blighted Ovum
Examinations conducted on pregnant women early in pregnancy will seldom detect a blighted ovum, even with ultrasound. It is only later, after the baby is supposed to have grown large enough to be visible (which should be by the sixth week of pregnancy), that a diagnosis of blighted ovum can be made.
The criterion for this diagnosis depends on what type of ultrasound is being used. Ordinarily a diagnosis of anembryonic pregnancy requires a mean gestational sac diameter (MGD) of 20–25 mm with no visible embryo. If a transvaginal ultrasound is used, however, the threshold for this diagnosis drops to 13 mm.
Treatment of a Blighted Ovum
A blighted ovum does not generally require treatment; the placenta will eventually stop growing on its own, and the woman’s body will expel any pregnancy-related tissue without medical intervention. If for some reason this does not happen, however, it may be necessary for the patient to undergo a procedure known as dilation and curettage (D&C) in order to remove the placental tissue and thereby prevent bleeding and infection.
Prevention of a Blighted Ovum
Generally speaking, a blighted ovum is a random occurrence, which therefore cannot be prevented. If your pregnancy ends in a diagnosis of blighted ovum, you can therefore take some comfort in the fact that it was not likely to have been caused by anything you did or did not do.
In most cases, women who experience an anembryonic pregnancy go on to have perfectly normal pregnancies on their next attempt, and deliver perfectly normal, healthy babies. It is rare for any woman to experience more than one anembryonic pregnancy. If your pregnancy ends in this way, your doctor will probably advise you to wait two or three menstrual cycles before attempting to become pregnant again.