A blighted ovum, also called an anembryonic pregnancy, happens when a fertilized egg implants in the uterus and a gestational sac develops, but an embryo never forms inside it. Chromosomal abnormalities are the primary cause, accounting for roughly half of all cases. The pregnancy may initially seem normal, with a positive test and early symptoms, but an ultrasound eventually reveals an empty sac. This is the most common cause of miscarriage in the first trimester.
Chromosomal Problems Are the Leading Cause
When a sperm fertilizes an egg, the resulting cell needs exactly the right number and arrangement of chromosomes to develop into an embryo. In about 50% of blighted ovum cases, the fertilized egg carries chromosomal abnormalities that prevent it from progressing past the earliest cell divisions. The placental tissue and gestational sac, which are less genetically demanding structures, can still grow for a time, but the embryo itself never takes shape.
The most common specific abnormality is trisomy 16, where cells have an extra copy of chromosome 16. This was found in 15% of anembryonic pregnancy samples in one chromosomal analysis. Trisomies of chromosomes 22, 15, and 19 were also identified, at rates of about 10%, 4%, and 4% respectively. Single autosomal trisomy (one extra copy of any non-sex chromosome) was the most prevalent category overall, making up 42% of the chromosomal errors found. These aren’t inherited conditions in most cases. They’re random errors that occur during cell division in the egg, sperm, or the first few divisions after fertilization.
Why the Sac Grows Without an Embryo
This is the part that confuses many people: if the pregnancy isn’t viable, why does anything grow at all? The answer is that the cells responsible for forming the placenta and the cells responsible for forming the embryo take slightly different developmental paths very early on. A fertilized egg divides into an outer layer of cells (which becomes the placenta and sac) and an inner cell mass (which becomes the embryo). In a blighted ovum, the outer layer manages to implant and begin building a sac, but the inner cell mass either fails to develop or stops growing almost immediately. Because the placental tissue is still active, it produces hCG, the hormone that triggers a positive pregnancy test and early pregnancy symptoms like nausea and breast tenderness.
Risk Factors That Increase the Odds
Most blighted ovums happen by chance, but certain factors raise the overall risk of early pregnancy loss.
Maternal age is the strongest predictor. The rate of early pregnancy loss is about 10% for women aged 20 to 24, and it climbs steeply with age, reaching 51% for women between 40 and 44. This increase is driven largely by the rising rate of chromosomal errors in eggs as women get older.
Obesity is another well-established risk factor for early pregnancy loss, though the exact mechanism linking it to anembryonic pregnancy specifically is less clear. It may involve hormonal imbalances or inflammatory changes that affect implantation and early embryo survival. Sperm quality also plays a role in some cases. Damage to sperm DNA can contribute to failed embryo development, though research connecting paternal factors to blighted ovum specifically is still limited compared to what’s known about egg quality.
How a Blighted Ovum Is Diagnosed
A blighted ovum is typically discovered during a routine early ultrasound, often between weeks 7 and 9 of pregnancy. The key finding is an empty gestational sac with no visible embryo. To avoid misdiagnosis (since very early pregnancies can look similar), providers use a specific size threshold: a mean sac diameter of 25 millimeters or greater with no embryo visible is considered definitive for anembryonic pregnancy.
If the sac is smaller than that, you’ll likely be asked to return for a follow-up ultrasound one to two weeks later to see whether an embryo has appeared. Blood tests tracking hCG levels can also help. In a healthy early pregnancy, hCG roughly doubles every 48 to 72 hours and peaks around weeks 8 to 10. In a blighted ovum, hCG levels often rise more slowly than expected, plateau, or remain abnormally low. Your provider may check levels over several days to see the trend.
What Happens After Diagnosis
Once a blighted ovum is confirmed, the pregnancy will end in miscarriage. Some women miscarry naturally (called expectant management), experiencing cramping and bleeding as the body passes the tissue on its own. Others prefer not to wait and opt for medical or procedural intervention.
Medical management uses medication to prompt the uterus to expel the pregnancy tissue. The largest U.S. trial on this approach found that about 71% of women had complete expulsion within three days of a single dose of vaginal medication. A two-medication protocol, which adds a second drug taken orally 24 hours before the first, significantly improves that success rate and reduces the likelihood of needing a surgical procedure afterward by roughly 63%.
Surgical evacuation (a brief procedure to remove the tissue) is the fastest and most predictable option, with the highest completion rate. It’s also recommended when there are signs of infection, heavy bleeding, or severe anemia. Most women recover physically within a few days after either approach.
Recovery and Returning Fertility
After a blighted ovum, most women have their first period within four to six weeks. Ovulation often returns before that first period, meaning it’s technically possible to conceive again within weeks of the loss.
The emotional recovery usually takes longer than the physical one, and there’s no standard timeline for when to try again. Physically, many providers consider it safe to try after one normal menstrual cycle, though individual circumstances vary.
Chances of It Happening Again
A single blighted ovum does not mean you’re likely to have another. The risk of miscarriage in a future pregnancy is about 24% after one loss, compared to 5 to 10% for women whose previous pregnancy was successful. After two consecutive miscarriages, the risk rises to about 30%, and after three or more, to roughly 35%. The vast majority of women who experience one blighted ovum go on to have healthy pregnancies.
If you’ve had two or more consecutive losses, chromosomal testing of both partners is sometimes recommended. In some cases, one partner carries a balanced chromosomal rearrangement that doesn’t affect their own health but increases the chance of chromosomal errors in embryos.
Can You Prevent a Blighted Ovum?
Because most blighted ovums result from random chromosomal errors, there’s no reliable way to prevent them. However, one nutritional factor does appear to lower the risk of early miscarriage in general. Women with folate deficiency (the natural form of folic acid) have a roughly 50% higher risk of miscarrying between six and twelve weeks. The standard recommendation is 400 micrograms of folic acid daily for all women of childbearing age, ideally starting before conception. Higher folate levels beyond the normal range don’t appear to offer additional protection, so more isn’t necessarily better.
Maintaining a healthy weight and managing chronic health conditions before pregnancy may also reduce the overall risk of early pregnancy loss, though neither can specifically prevent the chromosomal errors behind most blighted ovums.