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.
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.
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.
Obstetric Ultrasound is the use of ultrasound scans in pregnancy.
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 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 later in 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.