It can be unnerving to go in for an early pregnancy ultrasound and be told there's no fetal pole or an "empty gestational sac." Obviously it would be far more reassuring to see a baby developing right on schedule for what was expected, and ideally with a strong fetal heartbeat. But having no fetal pole does not automatically mean miscarriage.
How Incorrect Dates Can Explain Lack of a Fetal Pole
Although miscarriage is one possibility, the other possibility is that your pregnancy is simply not far enough along to see a fetal pole on an ultrasound. The fetal pole becomes visible in a transvaginal ultrasound somewhere between 5 to 6 weeks of gestational age, but any small error in dating the pregnancy can throw that off because the early pregnancy development varies so much from one day to the next.
If you incorrectly remembered your last menstrual period date, that can change the expected findings on an ultrasound -- or if you have an irregular cycle and do not ovulate two weeks after the start of your last menstrual period, your pregnancy may not technically be "five or six weeks along" even if it has been five or six weeks since your last menstrual period.
If you do not know your last menstrual period date, seeing no fetal pole may indicate the date of your pregnancy (and you may be given a rough estimated due date based on the measurement of the gestational sac).
When No Fetal Pole Means Miscarriage
Sometimes, sadly, an ultrasound finding of no fetal pole can be a sign of a miscarriage. This situation can be called a "blighted ovum;" a gestational sac develops but the baby never develops enough to become visible on an ultrasound. A blighted ovum can sometimes persist a number of weeks before any miscarriage symptoms appear, and the gestational sac may continue growing despite the lack of a baby.
If there is any possibility that the ultrasound shows no fetal pole because of an error in dating the pregnancy, the doctor will usually ask the woman to come in for another ultrasound in a few days or a week. This wait can be understandably difficult but can be necessary to avoid misdiagnosis of a miscarriage. If the problem was dating, the followup ultrasound should show continued development with a larger gestational sac and most likely the appearance of a fetal pole. If the followup ultrasound shows no development from the previous ultrasound, the doctor can conclusively diagnose miscarriage.
In a few cases, a miscarriage can be diagnosed from a single ultrasound that shows no fetal pole. These situations would include:
- A gestational sac larger than 16mm with no fetal pole
- Having falling hCG levels and no fetal pole
- Other evidence that rules out the possibility of incorrect dates, such as if the pregnancy was officially confirmed at 4 weeks gestational age and an ultrasound one month later showed development of only 5 weeks gestational age (especially alongside miscarriage symptoms)
American Pregnancy Association, "Concerns Regarding Early Fetal Development." Oct 2008. Accessed 3 Dec 2008.