An ultrasound is a highly accurate means of dating a pregnancy. In a normal pregnancy, ultrasound can provide an estimate of the gestational age to within five to seven days of accuracy. If a patient is having miscarriage symptoms, a doctor may order one or more ultrasounds to determine whether the pregnancy is viable.
How Early Pregnancy Ultrasound Works
In the first trimester, doctors usually use a transvaginal rather than abdominal ultrasound. The transvaginal ultrasound provides the most accurate information, given that the early developing gestational sac and fetal pole are extremely tiny at this point. A transvaginal ultrasound can get closer to the developing pregnancy. In a transvaginal ultrasound, the doctor or technician inserts a thin probe into the vagina. During the ultrasound, they take a series of measurements, including the size of the gestational sac, the size of the yolk sac, the length of the fetal pole, and the heart rate. In an abdominal ultrasound, the woman will be asked to come to the scan with a full bladder, because this positions the uterus in a way that makes it easier to obtain measurements. The doctor or technician then spreads a gel over the lower abdomen and uses a transceiver to take measurements from several angles.
Expected Ultrasound Results by Week in Early Pregnancy
Early Ultrasound for Determining Miscarriage
Ultrasound results are compared to what is expected for the gestational age of a pregnancy. The gestational age is calculated by the number of weeks since the last menstrual period; however, this method generally assumes a 28-day cycle with ovulation occurring on the 14th day. Many people have shorter or longer cycles and do not ovulate on the 14th day, and this could affect what an ultrasound should show in the development of the pregnancy. For example, if someone has a 35-day menstrual cycle, they most likely ovulate around the 21st day of their menstrual cycle (because ovulation usually occurs two weeks before the menstrual period would begin). If they became pregnant and had an ultrasound scan six weeks from their last menstrual period date, their normally developing pregnancy would measure with a gestational age of five weeks. If this person did not know that they ovulated on the 21st day of their cycle, they might worry unnecessarily that she was having a miscarriage. The early ultrasound would show only five weeks of development in the pregnancy, but the date of the last menstrual period would date the pregnancy at six weeks. Similarly, not everyone meticulously tracks the start of the menstrual period. If you cannot remember when your menstrual period started and guess the wrong day, even if you have a typical 28-day cycle, this could also change the expected results of an ultrasound scan.
When Results Are Uncertain
Sometimes an ultrasound will give uncertain results. For example, if a person is seven weeks pregnant, and the ultrasound does not reveal a fetal heartbeat, the doctor may order another ultrasound in a week. The pregnancy could still be normal but simply off by a few days in the dating. Or, the dating could be accurate, but still within the margin of error for when the heartbeat becomes detectable. Similarly, if an ultrasound reveals an empty gestational sac, this could still be a normal finding if the pregnancy is early along. The developing baby is too small to be seen on ultrasound until about five weeks of gestation. In this case, the doctor may opt to repeat the ultrasound at a later date. In either of these cases, the ultrasound results may indicate a miscarriage or the pregnancy may still be normal. If the subsequent ultrasound shows that the pregnancy has continued developing, the earlier results can be attributed to problems with dating. If the subsequent ultrasound still shows abnormal development, the doctor can conclusively diagnose a miscarriage. Often, the doctor cannot make a diagnosis based only on one early pregnancy ultrasound scan. The wait for the repeated ultrasound can be emotionally very difficult, but it may be necessary to avoid a misdiagnosis.
Other Diagnostic Information
If there is any question as to whether or not a person is having a miscarriage based on an ultrasound scan, the usual course of action is to do another ultrasound in a few days or a week. Sometimes, however, other diagnostic information can help a doctor interpret ultrasound results. For example, if you have a positive pregnancy test the day of your missed period, followed by confirmation with hCG blood test results, and an ultrasound four weeks later shows a pregnancy at only five weeks gestational age, the doctor may conclude that you have had a missed miscarriage. The positive pregnancy test a month earlier would indicate that the pregnancy should be more developed. In addition, if the ultrasound measurements indicate an embryo and/or heartbeat should be present and it is not, the doctor may diagnose miscarriage. The American College of Obstetricians and Gynecologists guideline is that if the crown-rump length is 7 mm or greater with no heartbeat, or if the mean sac diameter is 25 mm or greater and there is no heartbeat, pregnancy failure has occurred.
Ultrasounds in Late Pregnancy Loss
As pregnancy progresses, ultrasound becomes more and more accurate for determining the viability of a pregnancy. If an ultrasound in the second or third trimester shows that a baby has no heartbeat, this is considered conclusive for diagnosing a missed miscarriage or impending stillbirth. There was an error. Please try again.
