I just finished 10 days of Gonal-F injections followed by an HCG injection. Four "good" eggs developed by ultrasound, but conception did not occur. On the fourth day of bleeding an ultrasound showed two cysts in the ovary. I was told I must now skip at least the next cycle and must be followed closely. What is happening? Why wasn't this picked up on my daily ultrasound while I was getting the Gonal-F? I know this is not the same as ovarian hyperstimulation syndrome -- I do not feel that sick and am not bloated.

V.T.

As your doctor should have explained when your ultrasound was done, you have residual cysts left over from your last cycle. These cysts did not appear on your daily ultrasounds when you were getting your treatments because they are new -- they developed out of the follicles left when your ovaries released their eggs.

Residual cysts are one of the most frustrating problems associated with therapy with gonadotropin injections (Gonal-F, Follistim and others). These shots -- used to induce ovulation or to generate more eggs in women with unexplained infertility -- are usually administered for an average of eight to 10 days. The doctor periodically monitors the woman's ovaries with transvaginal ultrasound and tests of estrogen levels in the blood. The ultrasound can show the growth of follicles, the structures in the ovaries that hold eggs. When the ultrasound reveals the development of a certain number of sufficiently large (16-18mm) follicles, the doctor gives the woman a shot of the hormone hCG to prompt the follicles to release their eggs.

Once a follicle releases an egg, it is called a corpus luteum. This structure still plays a role in fertility, as it produces the hormone progesterone, which helps support a developing pregnancy if conception occurs. Normally, a corpus luteum disappears after about 14 days, unless you become pregnant. Sometimes the corpus luteum forms a small (2-3cm) cyst (fluid-filled mass). This happens quite often after ovulation treatment. Usually these cysts collapse and shrivel up by the end of the month, but occasionally (about 30 percent of the time) they hang around for another month or so before resolving on their own.

When you begin your next treatment cycle, you get a baseline ultrasound exam. If this exam shows the presence of residual corpora luteae, treatment will need to be delayed. These leftover cysts can diminish the effectiveness of fertility treatment and may lead to painful ovarian enlargement or torsion (twisting of the ovary that can result in loss of ovarian function).

Naturally, time is of the essence for most women wanting to conceive. Still, there is little to do but wait until the cysts disappear on their own. Luckily, this almost always happens within a month or two. In rare cases, treatment such as birth control pills will be needed to hasten the resolution. Rarer still, surgery is required.