Here’s why your pregnancy test may be wrong

Q: My home pregnancy test was positive, but the doctor did an ultrasound and said I had a “blighted ovum.” I’ve never heard of this. What is it and what does it mean for future pregnancies?

A: When applied to pregnancy, the term “blighted” sounds awful — cruel, even. Blighted ovum refers to an “anembryonic gestation” — a pregnancy that begins to grow in the absence of an embryo. This results in early miscarriage.  It is extremely common, occurring in up to 20 percent of known pregnancies.

The diagnosis of blighted ovum was made because you had an ultrasound exam. Without this technology, combined with your home pregnancy test, you might have thought your period was simply late and heavier than usual.

To understand the reason for the “blight,” let’s go back to the beginning of the pregnancy process — conception. When egg and sperm meet, each contributes half the chromosomes that complete the fertilized egg and let it form an embryo. For a pregnancy to be viable, these chromosomal units must meet, combine and replicate perfectly.

This complicated process is not always successful. More than 60 percent of first-trimester losses are due to an improper pairing of chromosomes.

However, even when the pairing is unsuccessful, trophoblastic cells (which create the placenta and cause pregnancy tests to be positive) can develop and grow. The pregnancy hormone (HCG), which these cells secrete, makes you miss a period. Levels of HGC may continue to rise, though not as fast as with a normal pregnancy, so you may initially feel many of the sensations of pregnancy, including breast tenderness and nausea. These hormones will also indicate a positive result on your pregnancy test.

Because a true pregnancy has not occurred, you eventually will start to bleed and pass the products of conception. This can occur from a few days to weeks after the initial positive pregnancy test

The ultrasound picture of a blighted ovum appears as an empty gestational sac — a round area within the uterus that is filled with clear fluid and nothing else. My patients often come to my office as soon as they do a home pregnancy test, wanting to confirm that they have a viable pregnancy. (One patient wanted to see her fetus before she had even missed her period!) Other patients are unsure of their dates and request an ultrasound to establish when they conceived.

Interpreting an ultrasound is not necessarily a one-step process. Sometimes it is performed too early to see the developing fetus, known in its earliest stages as the "fetal pole." If the fetal pole is not apparent, I check blood levels of HGC, and repeat this test after a few days. In a normal pregnancy, the level nearly doubles on a daily basis.

Generally, I will not make a final diagnosis of blighted ovum until I have checked additional hormone levels and performed a second ultrasound a week later to see that the sac remains empty.

Once this confirms the diagnosis, there are a few options.

  • One is to wait and see whether the patient’s period starts and she bleeds the remaining trophoblastic tissue out. Few women wish to follow this course of inaction.
  • Most women opt for a D & C (dilation and curettage), a routine surgical procedure that scrapes the remaining products of conception out of the uterus.
  • A medication is administered to terminate pregnancy, which causes sloughing of the uterine lining.

There is a slight chance, about 1 in 250, that a patient who does not show a normal sac with a fetal pole in her initial ultrasound could have a tubal or ectopic pregnancy, so follow-up with additional ultrasound is important.

Blighted ovum is usually unique to a particular pregnancy. In most cases, a woman who receives this diagnosis will go on to have a healthy, normal pregnancy.

For older women, the risk of miscarriage, including blighted ovum, increases, probably because older eggs have been spending lots of time incurring chromosomal damage. If this type of early miscarriage does recur, especially if you are under 35, your doctor may want to do genetic blood tests to look at possible chromosomal abnormalities in you and your partner.

Dr. Reichman’s Bottom Line: If your pregnancy ends early due to a blighted ovum, don’t despair. If you try again, you will probably have a successful pregnancy.

Dr. Judith Reichman, the “Today” show's medical contributor on women's health, has practiced obstetrics and gynecology for more than 20 years. You will find many answers to your questions in her latest book, "Slow Your Clock Down: The Complete Guide to a Healthy, Younger You," published by William Morrow, a division of .

PLEASE NOTE: The information in this column should not be construed as providing specific medical advice, but rather to offer readers information to better understand their lives and health. It is not intended to provide an alternative to professional treatment or to replace the services of a physician.