Gonadotropin Releasing Hormone (GnRH): Its Role in Fertility and the Menstrual Cycle
Introduction
Understanding how hormones govern the female reproductive cycle is fundamental to appreciating both natural fertility and assisted reproduction treatments such as IVF. At the heart of this hormonal cascade sits Gonadotropin Releasing Hormone, commonly known as GnRH. According to EmBIO Medical Centre in Athens, one of Greece's leading fertility centres, GnRH plays a central and indispensable role in coordinating the complex events of the menstrual cycle.
What Is GnRH and Where Does It Come From?
The first menstrual cycle begins at puberty, officially marking the start of a woman's reproductive period. Each normal cycle lasts approximately 28 days, although considerable individual variation exists in actual duration. The cycle is governed by different hormones secreted by the hypothalamus, the pituitary gland (glands of the brain), and the ovaries.
At the beginning of each cycle, the hypothalamus secretes Gonadotropin Releasing Hormone (GnRH) in pulses. This hormone acts upon the pituitary gland, which in turn releases two key hormones: FSH (Follicle Stimulating Hormone) and LH (Luteinising Hormone).
GnRH therefore acts as the master switch, the signal that sets the entire reproductive process in motion. Without this initial hormonal trigger from the hypothalamus, the subsequent chain of events leading to ovulation and potential conception cannot occur.
The Purpose of the Hormonal Cascade
The purpose of these hormones is to stimulate the ovaries to release a mature egg and to create a suitable environment in the uterus to support a potential pregnancy.
Once GnRH has triggered the release of FSH and LH from the pituitary gland, these two hormones work in harmony, each with a distinct and vital role, to prepare the body for ovulation and, ultimately, fertilisation.
FSH and LH: The Two Key Gonadotrophins
These hormones display a harmonious relationship with one another and act upon the ovary for different reasons. FSH is responsible for the creation and development of follicles (small fluid-filled cysts, each containing one egg), whilst LH is responsible for the final maturation of these follicles and for triggering ovulation.
The two phases of the menstrual cycle reflect the sequential action of these hormones.
The Follicular Phase (Days 1–14)
Under the influence of FSH, approximately 8–10 follicles begin to develop within the ovary. Normally, however, only one of these prevails and reaches full maturity. The developing follicles produce oestrogens, and in particular oestradiol (E2). Oestradiol levels rise as the number and size of the developing follicles increase.
The secreted oestradiol acts upon the endometrium (the inner lining of the uterus), increasing its thickness. At the same time, it widens the cervical canal and acts upon the cervical mucus, causing the secretion of a more alkaline mucus which favours the survival and transport of sperm.
Oestradiol is therefore the hormone that assists the upward journey of sperm from the vagina towards the fallopian tube, and prepares the endometrium to receive the embryo.
The Luteal Phase and Ovulation
On days 13 to 14 of the cycle, a sudden surge in LH triggers the final maturation of the follicle and the release of the egg ovulation. The mature egg is collected by the fimbriae (the outermost section of the fallopian tube) and is propelled along it.
After ovulation, the remnants of the follicle are transformed into the corpus luteum. The corpus luteum secretes large quantities of progesterone, which prevents the shedding of the endometrium and provides hormonal support during the early weeks of pregnancy. The role of the corpus luteum is taken over by the placenta after the twelfth week of pregnancy.
If pregnancy is not achieved, the corpus luteum breaks down after a few days and the secretion of FSH and LH decreases significantly.
The Process of Conception
The egg survives in the fallopian tube for approximately 24–36 hours and is ready for fertilisation. If sexual intercourse takes place during this window, sperm that reach the fallopian tube will come into contact with the egg, and one sperm will penetrate it, achieving fertilisation. Under normal circumstances, only a single sperm enters the egg.
The fertilised egg (zygote) begins cellular division whilst simultaneously moving towards the uterus, assisted by the peristaltic movements of the fallopian tube. The embryo remains in the fallopian tube for the following 5–6 days, and upon reaching the uterine cavity it consists of 60–120 cells, this is known as the blastocyst stage. At this point, it implants into the endometrium and continues its development.
Why GnRH Matters in Fertility Treatment
Understanding the pivotal role of GnRH in the menstrual cycle helps explain why it is such a critical target in fertility medicine. The entire process of follicular development, ovulation, and endometrial preparation depends on the pulsatile release of GnRH from the hypothalamus, which drives the release of FSH and LH in a precisely timed sequence. Any disruption to this hormonal axis can impair fertility.
At EmBIO Medical Centre, the team of fertility specialists, led by Dr Thanos Paraschos, a gynaecologist, obstetrician, and Director of the Assisted Reproduction Centre, provides thorough assessment and treatment of hormonal and fertility-related conditions. The centre offers a full range of fertility treatments, from conventional IVF and egg donation to natural cycle IVF and intrauterine insemination (IUI), all underpinned by a deep understanding of the hormonal processes that govern reproduction.
