Human Chorionic Gonadotropin (HCG) is a naturally occurring hormone produced by the developing placenta. In fertility treatments, specifically In Vitro Fertilization (IVF), HCG mimics Luteinizing Hormone (LH) to control egg maturation. While HCG is most famously used in a single, high-dose injection, this article focuses on a distinct application: the use of low-dose HCG to support the uterine environment after embryo transfer. This approach aims to enhance the chance of successful implantation and support early pregnancy.
HCG’s Role in Standard IVF Protocols
The most common application of HCG in an IVF cycle is the high-dose “trigger shot,” administered approximately 36 hours before egg retrieval. This injection, typically 5,000 to 10,000 International Units (IU) of HCG, finalizes egg maturation within the ovarian follicles. HCG mimics the body’s natural LH surge, signaling the eggs to complete the final cell division process in preparation for fertilization. The precise timing of this injection dictates the scheduling of the egg retrieval.
Defining Low-Dose HCG as Luteal Phase Support
Low-dose HCG (LD-HCG) is used for Luteal Phase Support (LPS), a treatment given after egg retrieval and embryo transfer to ensure the uterine lining is receptive. The luteal phase relies on the corpus luteum, the structure left behind in the ovary after the egg is released, to produce progesterone and estrogen. In an IVF cycle, aspirating the follicular fluid during egg retrieval can disrupt the granulosa cells, which are part of the corpus luteum. Because of this disruption, patients undergoing IVF almost always require external hormonal support. LD-HCG is sometimes used as an alternative or an adjunct to conventional progesterone and estrogen supplements. It stimulates the remaining corpus luteum tissue, encouraging the body’s own natural hormone production to support the endometrium for implantation.
Mechanism of Action in Endometrial Receptivity
HCG and LH share the same receptor (LHCGR), which is present on corpus luteum and endometrial cells. By providing a low, sustained dose, LD-HCG stimulates the existing corpus luteum to increase its production of endogenous progesterone and estradiol. This is a more physiological way to support the luteal phase compared to relying solely on external progesterone supplements.
LD-HCG also has direct effects on the endometrium, the lining of the uterus. HCG acts on endometrial cells to promote decidualization, the necessary transformation of the uterine lining to prepare for and sustain a pregnancy. The hormone helps increase local vascularity by stimulating the release of factors like Vascular Endothelial Growth Factor (VEGF), which is important for the formation of new blood vessels needed for a developing placenta. HCG may also modulate local immune responses at the maternal-fetal interface, helping to create a welcoming environment for the embryo.
Administration and Monitoring
LD-HCG is typically administered via subcutaneous injection, starting a few days after egg retrieval or fresh embryo transfer. The dosage is significantly lower than the trigger shot, often ranging from 100 to 1,500 IU, and may be given daily or every few days during the luteal phase. Protocols vary, using doses like 100 IU daily or 1,500 IU every two to three days.
The patient’s response is monitored through blood tests, primarily measuring serum levels of progesterone and estradiol. These tests ensure the corpus luteum is adequately stimulated to produce sufficient hormones. If hormone levels are outside the desired range, the protocol may be adjusted by increasing the dose or adding conventional progesterone and estrogen supplementation. HCG carries an increased risk of Ovarian Hyperstimulation Syndrome (OHSS), even at lower doses, particularly in high-responding patients.