Symptoms of ovarian hyperstimulation syndrome often begin within 10 days after using injectable medications to stimulate ovulation. Symptoms can range from mild to severe and may worsen or improve over time.

Mild to moderate OHSS

With mild to moderate ovarian hyperstimulation syndrome, symptoms can include:

• Mild to moderate abdominal pain
• Abdominal bloating or increased waist size
• Nausea
• Vomiting
• Diarrhea
• Tenderness in the area of your ovaries
• Sudden weight increase of more than 6.6 pounds (3 kilograms)

Some women who use injectable fertility drugs get a mild form of OHSS, which goes away after about a week. If pregnancy occurs, however, symptoms of OHSS may worsen and last several days to weeks.

Severe OHSS

With severe ovarian hyperstimulation syndrome, you might have:

• Rapid weight gain — such as 33 to 44 pounds (15 to 20 kilograms) in five to 10 days
• Severe abdominal pain
• Severe, persistent nausea and vomiting
• Blood clots in legs
• Decreased urination
• Shortness of breath
• Tight or enlarged abdomen

The cause of ovarian hyperstimulation syndrome isn't fully understood, although having a high level of human chorionic gonadotropin (HCG) — a hormone usually produced during pregnancy — introduced into your system plays a role. Ovarian blood vessels react abnormally to HCG and begin to leak fluid. This fluid swells the ovaries, and sometimes large amounts move into the abdomen.

During fertility treatments, HCG may be given as a "trigger" so that a mature follicle will release its egg. OHSS usually happens within a week after you receive an HCG injection. If you become pregnant during a treatment cycle, OHSS may worsen as your body begins producing its own HCG in response to the pregnancy.

Injectable fertility medications are more likely to cause OHSS than is treatment with clomiphene, a medication given as a pill you take by mouth.

Factors that increase your risk of OHSS include:

• Polycystic ovary syndrome — a common reproductive disorder that causes irregular menstrual periods, excess hair growth and unusual appearance of the ovaries on ultrasound examination
• Large number of follicles
• Age under 30
• Low body weight
• High or steeply increasing level of estradiol (estrogen) before an HCG trigger shot
• Previous episodes of OHSS

In some cases, OHSS affects women who have no risk factors at all.

About 1 to 2 percent of women undergoing ovarian stimulation develop a severe form of ovarian hyperstimulation syndrome. Severe OHSS can be life-threatening. Complications may include:

• Fluid collection in the abdomen and sometimes the chest
• Electrolyte disturbances (sodium, potassium, others)
• Blood clots in large vessels, usually in the legs
• Kidney failure
• Twisting of an ovary (ovarian torsion)
• Rupture of a cyst in an ovary, which can lead to serious bleeding
• Breathing problems
• Pregnancy loss from miscarriage or termination because of complications
• Rarely, death