During a normal menstrual cycle an egg is released from the ovary at the time of ovulation. Following release from the ovary, the egg is usually picked up by the Fallopian tube and can survive there for up to 2 days. Understanding how your cycle works and carefully monitoring it can help improve your fertility chances.
How does the menstrual cycle come about?
Effect of hormones on the womb lining during the menstrual cycle:
The menstrual cycle reflects changes in the levels of circulating hormones, which act on the lining of the womb to cause it to become thickened and then to break down (the menstrual bleed) if pregnancy does not occur. The two hormones that act directly on the womb lining are oestrogen and progesterone. Both hormones are produced by follicles in the ovary, which also contain eggs. Oestrogen causes the lining to become thick. Progesterone blocks the effect of oestrogen and causes the lining to become “secretory”.
Follicle development and ovulation:
Follicles are little “chambers” that house eggs within the ovary. Each follicle contains one egg and undergoes a long period of development in order to produce an egg capable of supporting pregnancy. The image below shows the various stages of follicle development. At the extreme left is the earliest stage known as the primordial follicle stage. After growing for around 3 months in humans, primordial follicles develop to the “antral” stage; so-named because at this stage follicles have developed a small sac of fluid called an antrum (shown in blue). Growth of antral follicles is controlled by the hormone, FSH, which is produced by a small gland at the base of the brain called the pituitary gland. Under the influence of FSH, antral stage follicles produce oestrogen, the levels of which increase as antral follicles enlarge. When the antral follicle reaches around 2cm in diameter, it is induced to rupture and release its egg by another pituitary hormone called LH. This is known as ovulation and is shown at the extreme right of the image below.
Putting it all together – Hormones, follicles, ovulation and womb lining:
At the start of a menstrual cycle, FSH levels begin to increase causing a group of small antral follicles to begin developing. One follicle soon takes the lead and becomes the dominant antral follicle. As the dominant follicle grows under the influence of FSH, it produces more and more oestrogen that causes the lining of the womb to thicken or “proliferate”. Eventually, high oestrogen levels cause the pituitary gland to release a surge of LH (the LH surge), which causes the follicle to rupture and release its egg (see image above). The follicle from which the egg was released then becomes a corpus luteum and now starts producing the other major female hormone, progesterone. Progesterone causes different effects on the womb lining from oestrogen; progesterone blocks the effect of oestrogen and causes the lining to become “secretory”, which helps the embryo to implant and make a pregnancy. If pregnancy does not occur, progesterone eventually causes the lining to be shed as the menstrual bleed.
When is the fertile time of the menstrual cycle?
The fertile time of the cycle coincides with when the egg has been released from the ovary and it is sitting within the end portion of the Fallopian tube. This is usually around 2 weeks before the start of the next menstrual bleed. This means that for a typical 28-day cycle, the fertile time is around 14 days after the bleed started (Day 14). The fertile time will differ depending on cycle length. For instance, for a 32-day cycle, the fertile time is around Day 18. The fertile time, and a couple days either side of it, are the best times for having sexual intercourse in order to optimise the chances of becoming pregnant.
How can I detect when ovulation will occur?
The most accurate way to determine the timing of ovulation is to track ovarian follicle development using ultrasound scanning. In this way, follicle growth can be directly seen with ultrasound and the precise timing of the LH surge determined using blood tests. This is called “cycle tracking” or “ovulation tracking”. Ultrasound scanning also provides other very valuable pieces of information, for instance, about ovarian reserve (the numbers of eggs in the ovary) and whether or not the womb lining is developing properly during the cycle. It will also identify other abnormalities in the womb that might affect pregnancy success such as fibroids and polyps.
Another option is to monitor for the LH surge in the urine. This is how ovulation detector kits work. When LH levels surge in the blood, LH is expected to enter the urine and to be detected using the kit.
The egg is released 1-2 days after the LH surge, so intercourse around the time of the LH surge will increase the chances for sperm to interact with the egg.
How can I get help monitoring my cycle to improve pregnancy chances?
A fertility specialist can help you by monitoring the development of follicles in the ovary and the lining of the womb using ultrasound and blood tests. This will also identify any potential problems that may need to be treated to improve pregnancy chances. Prof Homer has extensive experience with cycle monitoring and optimising fertility prospects.