During a woman’s natural menstrual cycle, one egg develops within a chamber called a follicle. After the follicle has developed to an advanced stage, it ruptures to release a mature egg from the surface of the ovary, termed ovulation. Growth of the follicle occurs under the influence of the hormone, Follicle Stimulating Hormone (FSH), that is produced by a very small gland at the base of the brain called the pituitary gland.
One of the main objectives during IVF is to produce multiple eggs (not just one egg as during a natural cycle). To achieve this, FSH is administered by daily injection under the skin of the tummy, usually for around 8-14 days; this is referred to as ovarian stimulation. FSH and other IVF medications are available in easy-to-use “pen” format.
During this time follicle development is monitored using transvaginal ultrasound scanning and blood tests for measuring the levels of the hormone, oestrogen. With a good response, more than 3 follicles will grow beyond 10 mm at the rate of roughly 1-2 mm per day and since follicles produce the majority of the circulating oestrogen, oestrogen levels will also rise. Increasing levels of oestrogen are sensed by the pituitary and can cause the pituitary to release the hormone, luteinising hormone (LH), in the form of a so-called “LH surge” which triggers ovulation. During IVF, it is important to avoid such an LH surge since this would cause the eggs to be released within the body prior to their retrieval. To prevent the surge, an LH “blocker” is also used. Typically, this is used in the form of an “antagonist” medication that is introduced around the fifth day of FSH injections.
When follicles have become large enough (typically around 1.7-2 cm in diameter), this is a sign that the egg inside the follicle can be induced to “mature”. This maturation process is brought about by the “trigger” injection, which is usually human chorionic gonadotrophin (hCG) hormone. The hCG trigger has the same effect as the body’s LH surge and is given around 36 hours prior to planned egg pick-up. Since only mature eggs can be fertilised by sperm, without the trigger, there will be no mature eggs and the IVF cycle will be unsuccessful.