Pregnancy dating and viability
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).
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.
“Induced Abortion.” The American College of Obstetricians and Gynecologists.
In RA Hatcher et al., eds., Contraceptive Technology, 17th , pp.511-544.
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 procedures that can be done in the third trimester include: Compiled using information from the following sources: 1. American Journal of Obstetrics and Gynecology, 183 (2): s54-s64.
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.
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.
Medical abortions are only available up through nine weeks gestation.