A little anatomy to get us started
The vagina is a fibromuscular elastic tube which extends from the vulva at the outer end to the neck of the uterus or cervix. The function of this female organ is to receive the penis during intercourse and also to allow menstrual blood to escape, as well as being the (lower) birth canal during childbirth.
The uterus, also known as the womb, is the organ in which the pregnancy develops. It is the largest part of the female reproductive system. It is a hollow, muscular, pear-shaped organ, located between the cervix and the Fallopian tubes. It is lined on the inside with a membrane known as the endometrium.
The Fallopian tubes are muscular tubes which connect the uterus or womb to the ovaries.
The ovaries are two walnut-sized organs, and this is the part of the female reproductive system which is responsible for producing and releasing the sex hormones and ova. They are almond-shaped structures measuring 1x2x3 cm in fertile women (although their size changes during the cycle). They weigh approximately 6 - 7 grams, and are greyish-white in colour. They are attached to both sides of the uterus by the uterine-ovarian ligaments and to the pelvic wall by the infundibulopelvic ligament. The ovaries in women are equivalent to testicles in men.
Ova are female sex cells (or female gametes). They are microscopic cells (measuring about 80 microns, although they are much larger than sperm cells) which are spherical in shape and which cannot move.
Each month (approximately every 28 days), between puberty and the menopause, several ova begin to mature in the ovaries inside small fluid-filled “cysts” called primordial follicles, in a process known as oogenesis. Only one of these follicles (containing an ovum) will become dominant, while the others become atrophied and are absorbed by the ovary, and are not reused in future cycles.
Towards the middle of the woman’s menstrual cycle, the dominant follicle will release a single ovum at the point of ovulation, which travels along the Fallopian tube towards the uterus.
The Ovarian Reserve
The number or ova, or of primordial follicles, which amounts to the same thing, that a woman has is called the ovarian reserve, and this is determined before her birth. In fact, the point at which a woman has the most primordial follicles (around 7 million follicles) is 20 weeks before she is born.
From that moment onwards, the number of follicles decreases as time goes on, and these are not replaced by new ones. At birth a woman has approximately 2 million primordial follicles, and she has around 400,000 by the time she has her first period.
Today, the assessment of a woman’s ovarian reserve is vitally important to a reproductive specialist in terms of evaluating a patient’s reproductive prognosis. This assessment is done using extremely useful data such as high-resolution ultrasound counts of antral follicles, and hormonal evaluation from hormones such as FSH and Anti-Mullerian hormone (AMH).
The Menstrual Cycle and Ovulation
The pituitary gland in the brain is responsible for producing and regulating the hormones which control the sequence of events which lead to ovulation. The two hormones which govern this process are:
- Follicle-stimulating hormone (from here on FSH)
- Luteinizing hormone (from here on LH)
The way in which these hormones are released allows the menstrual cycle to be divided into three phases:
- Follicular phase: This begins on day 1 of the menstrual cycle, which is the first day of menstrual bleeding. FSH begins stimulating the development of several follicles, but as levels of FSH fall gradually in a natural cycle, just one follicle will become dominant and go on to produce a mature ovum. The non-dominant follicles are absorbed by the ovary and cannot be reused in future cycles (in other words, they are lost). The developing follicle also secretes oestrogens, which, amongst other functions, are important for bringing about the necessary changes in the cervical mucus for enabling sperm to pass through the uterus, and for causing the thickening of the endometrium which is required for the embryo to become implanted. The main oestrogen produced by the ovaries is estradiol (E2).
- Ovulatory phase: This is a short phase. It begins when, in response to rising oestrogen levels, the level of LH increases dramatically. This peak in LH triggers the final maturation of the ovum, causing the follicle to rupture and the ovum to be released. This normally occurs 14 days before the following menstruation, which is equivalent to day 14 of a 28-day cycle.
- Luteal phase: This begins after ovulation. At this point, the ovarian follicle which released the ovum during the previous phase collapses and solidifies and becomes what is known as a corpus luteum. This structure is very important as it produces progesterone, which is the hormone responsible for transforming and preparing the lining of the uterus, so that a fertilised egg (the early embryo) can become implanted and begin to develop. If conception does not take place, the corpus luteum stops working round about day 26 of the cycle (in a 28-day cycle). Without the support of progesterone, the endometrium begins to break down and is removed from the body during menstruation.