Fertility Treatment: What is an HCG Trigger Shot?

If you are undergoing in vitro fertilization (IVF), intrauterine insemination (IUI), or using timed intercourse methods, your fertility plan may involve the injection of a hormone called human chorionic gonadotropin (hCG).

What is human chorionic gonadotropin (hCG)

human chorionic gonadotropin is a hormone produced by a fertilized egg. In pregnant women, the placenta produces hCG naturally after implantation (in fact, pregnancy tests look for it in the blood). It's also similar in chemical structure and function to luteinizing hormone (LH)–the hormone that triggers ovulation–and fertility doctors sometimes use hCG before pregnancy too to help ensure the proper and complete release of eggs.

Getting the timing right

The hCG trigger shot is all about timing. Your doctor may use ultrasound and blood tests to determine when the ovarian follicles are mature and the injection should take place. If the timing of the injection is successful, the mature eggs should be available for fertilization through intrauterine insemination (IUI) or timed intercourse, or for harvesting for in vitro fertilization (IVF). In most cases, patients are asked to give themselves the injection.

Egg donors are also usually asked to have a hCG trigger shot to make sure all the mature eggs are released on schedule.

Ovulation generally happens somewhere between 34 and 46 hours after the shot is given — most commonly about 36 hours afterwards.

Couples using the timed-intercourse method of conception may be advised to have more than one session of intercourse in order to get the best chances of fertilizing the egg in conjunction with a hCG trigger shot, for example a day or two before ovulation and then on the day of ovulation.

hCG trigger shot side effects and risks

Sometimes side effects of an hCG trigger shot may include cramping and bleeding. Patients may also experience other symptoms like stomach upset, headache, fever, joint pain, or an allergic reaction.

Risks include:

  • False positives. The hCG stays in the urine for a while, which can lead to false positives on pregnancy tests and other routine urine testing if given less than 10 days after the shot.
  • Ovarian hyperstimulation syndrome (OHSS). Symptoms of OHSS include severe abdominal pain with sudden onset, vomiting, and thirst or signs of dehydration. Although most cases of OHSS aren't serious, anyone with the symptoms needs to get medical attention right away, because OHSS can occasionally be severe and even life-threatening.
  • Multiple births. Women with polycystic ovary syndrome (PCOS) face an increased (one in three) probability of having multiple babies when taking fertility medications such as hCG.

Talking to your doctor

It's important for you and your doctor to discuss all the risks and benefits of the hCG trigger shot, as well as potential side effects and complications. Give your doctor a complete and accurate medical history — as well as a list of all known allergies — in order to lessen the risk of complications.

Updated August 2014

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