Understand Early Pregnancy Ultrasound Results

How Early Pregnancy Ultrasound Works

Transvaginal Ultrasound
Most early pregnancy ultrasounds are performed with transvaginal ultrasound probes.. Image © A.D.A.M.

In the first trimester, doctors usually use a transvaginal rather than abdominal ultrasound to gather information about the pregnancy. The transvaginal ultrasound provides the most accurate information in early pregnancy, given that the early developing gestational sac and fetal pole are extremely tiny at this point and a vaginal ultrasound can get closer to the developing pregnancy.

In a transvaginal ultrasound, the doctor or technician inserts a thin probe into the vagina in order to 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 a number of angles.

Why Doctors Order Ultrasound in Early Pregnancy

Early Pregnancy
In early pregnancy, the ultrasound is attempting to get an image of a very small gestational sac.. Image © A.D.A.M.

Researchers have mapped out the expected measurements for specific points in early pregnancy, so an early ultrasound scan can give a doctor some important information.

A common use of ultrasound in early pregnancy is to calculate a gestational age. For this purpose, an ultrasound is considered a highly accurate means of dating a pregnancy. In a normal pregnancy, the ultrasound results can provide an estimate of the gestational age to within five to seven days of accuracy.

When a woman is having miscarriage symptoms, a doctor may order one or more ultrasounds to determine the viability of the pregnancy.

Why Early Ultrasound Results Can Be Inconclusive for Determining Miscarriage

Ultrasound Image with Fetal Heartbeat
Ultrasound images are typically very grainy in quality.. Image © A.D.A.M.

The results of an ultrasound 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 women 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 a woman has a 35-day menstrual cycle, she most likely ovulates around the 21st day of her menstrual cycle (because ovulation usually occurs two weeks before the menstrual period would begin). If the woman became pregnant and had an ultrasound scan six weeks from her last menstrual period date, her normally developing pregnancy would measure with a gestational age of five weeks, because the gestational age dating system assumes that she would have ovulated a week earlier than she did. If the woman did not know that she ovulated on the 21st day of her cycle, she might worry unnecessarily that she was having a miscarriage if she had an early ultrasound that showed only five weeks of development in her pregnancy when she was technically "six weeks pregnant."

Similarly, not everyone meticulously tracks the start of the menstrual period. If a woman cannot remember when her menstrual period started and guesses the wrong day, even if she does have a typical 28-day cycle, this could also change the expected results of an ultrasound scan.

In early pregnancy, the gestational sac and embryo change drastically every day, so being off even by a few days with dating can make a difference in whether or not the ultrasound should detect a heartbeat.

How hCG Results or Other Information Can Help in Ultrasound Interpretation

If there is any question as to whether or not a woman is having a miscarriage based on an ultrasound scan, the usual course of action is that the doctor will order another ultrasound in a few days or a week.

Sometimes, however, the presence of other diagnostic information can help a doctor to interpret ultrasound results even if there is only one ultrasound scan. For example, if a woman has a positive pregnancy test the day of her 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 the woman has had a missed miscarriage -- because the positive pregnancy test a month earlier would indicate that the pregnancy should be more developed.

In addition, if the ultrasound measurements indicate a fetal pole and/or heartbeat should be present and it is not, the doctor may diagnose miscarriage. The American Pregnancy Association cites the guideline that if the gestational sac is larger than 16-18 millimeters and contains no fetal pole or if the fetal pole is larger than 5 millimeters and has no heartbeat, miscarriage has occurred. (These situations would be called a blighted ovum or missed miscarriage, respectively.)

Expected Ultrasound Results By Week of an Early Pregnancy

Ultrasound Findings in Early Pregnancy
Info Source: American Pregnancy Association. Image Created By Krissi Danielsson

These are averages for when specific early pregnancy developmental points become detectable by transvaginal ultrasound. Generally, abdominal ultrasound is less sensitive and may not detect these milestones until a week or more later in the pregnancy.

Remember also that any uncertainty about the date of ovulation could affect what the ultrasound will show at any point in early pregnancy.

Empty Gestational Sac / No Fetal Heartbeat / No Fetal Pole - Is There Hope?

Abdominal Ultrasound
Abdominal ultrasounds are used in later pregnancy but can yield less accurate results in early pregnancy.. Image © A.D.A.M.

Sometimes an ultrasound will give uncertain results. For example, if a woman 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 the timing is still within the margin of error for when the heartbeat becomes detectable on an ultrasound.

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 -- but the doctor cannot determine the result based only on one early pregnancy ultrasound scan in many cases.

The wait for the repeated ultrasound can be emotionally very difficult, but it may be necessary in order to avoid a misdiagnosis unless other information is present to help the doctor interpret the ultrasound results.

Ultrasounds for Confirming Late Pregnancy Loss

The information in this walkthrough is specific to ultrasounds done in early pregnancy, particularly in the early first trimester before seven weeks of pregnancy. As the pregnancy progresses, the 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.


American Pregnancy Association, "Concerns Regarding Early Fetal Development." May 2007. Accessed 26 May 2008.

Savitz, David A., James W. Terry, Nancy Dole, John Thorp, Anna Maria Siega-Riz, Amy H. Herring, "Comparison of pregnancy dating by last menstrual period, ultrasound scanning, and their combination." American Journal of Obstetrics & Gynecology 2002. Accessed 26 May 2008.

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