Q: How does an ultrasound detect an ectopic pregnancy?

A: An early intrauterine pregnancy is relatively easy to see with high resolution transvaginal ultrasound scanning, but a pregnancy outside the uterus (ectopic pregnancy) is more difficult to see.

The appearance of the ectopic pregnancy itself is the same as for intrauterine pregnancies. Depending of the gestational age and normalcy of development, you may see a gestational sac, a yolk sac, a fetal pole, and a fetal heartbeat. The difficulty lies in finding the pregnancy without the normal uterine landmarks.

Using transvaginal scanning, about half of the ectopic pregnancies have only an indirect evidence of an ectopic pregnancy, while in the other half one can see the pregnancy directly outside the uterus.

The indirect evidence of an ectopic pregnancy includes:

• Absence of an identifiable intrauterine pregnancy with maternal serum HCG levels of more than 1500 (this number varies and may be lower in some labs).
• Presence of an intrauterine gestational "pseudosac." These thin-walled structures represent some fluid (sometimes blood) within a decidualized endometrium that bears a superficial resemblence to a gestational sac. However, it lacks the bright echogenic ring of a true gestational sac and will never contain a yolk sac.
• Large amounts of free fluid (blood) inside the abdominal cavity. Small amounts of free fluid are non-diagnostic, as this is commonly seen in cases of spontaneous abortion, ruptured ovarian cysts, and ovulation.