Stages of conception
For a pregnancy to be successful there are several stages of a process that must be achieved. These include:
- Ovulation, which is the release of the egg from the ovary.
- Passage of the egg through the fallopian tube into the uterus.
- Fertilisation of the egg by the sperm, which usually takes place as the sperm meets the egg on its way through the fallopian tube towards the uterus.
- Embryo development from the fertilised egg (now the zygote), which remains in the fallopian tube for 2-3 days and begins to divide forming the embryo.
- Implantation of the embryo at day 5-7 post fertilisation into the endometrial lining of the uterus to form the placenta.
Causes of infertility
Infertility is defined by the World Health Organisation as the inability to become pregnant after one year of unprotected intercourse. In about 30% of infertility cases the problem lies with the woman and this can be the result of several factors:
- Ovulation failure (anovulation) due to hormonal problems, such as overproduction of testosterone (usually associated with polycystic ovary syndrome), or over production of prolactin by the pituitary gland, which suppresses oestrogen production by the ovaries.
- Luteal phase defect, which is due to insufficient production of progesterone by the ovaries, and can result in failure of the endometrium to develop sufficiently for implantation.
- Blocked or damaged fallopian tubes (salpingitis) which prevents the egg from becoming fertilised and reaching the uterus. This could be due to pelvic inflammatory disease (usually caused by a sexually transmitted disease) endometriosis, adhesions, or surgery for ectopic pregnancy.
- Endometriosis can be the cause of infertility and is the growth of endometrial tissue outside the uterus, which can affect the normal functioning of the female reproductive system.
- Problems with the uterus, which can be a defect in the shape or the growth of benign tumours called fibroids.
- Treatments for cancer, particularly if they affect the cervix, uterus, endometrium or other parts of the female reproductive system.
- Certain medical conditions that can affect the normal menstrual cycle, such as kidney disease, thyroid problems or diabetes.
- Early menopause related to early ovarian failure.
Ovulation
Ovulation is dependent on a series of hormonal interactions that result in the release of an egg from the ovary.
Oestrogen levels produced by the ovary are low during the first phase of the menstrual cycle; known as the follicular phase and when oestrogen levels drop this induces the release of FSH (follicle stimulating hormone) from the hypothalamus, which stimulates growth and maturation of the ovarian follicle that releases the egg. As the ovarian follicles mature, oestrogen levels rise and this induces the release of LH (luteinizing hormone) from the pituitary gland.
Both LH and FSH are under the control of regulating hormones produced by the hypothalamus called gonadotropin releasing hormones (GnRH). An intricate feedback mechanism exists between the hormones involved in regulating the menstrual cycle and if any part of the interaction of hormones malfunctions, this can prevent ovulation a condition known as anovulation
When LH reaches a peak, known as the LH surge, this stimulates rupture of the dominant follicle releasing a mature egg and this is ovulation. The other follicles die down and the active ruptured follicle becomes the corpus luteum and starts producing progesterone, which induces the endometrium (lining of the uterus) to thicken and ripen ready for conception.
Treatments to restore ovulation
One treatment for infertility due to anovulation
is the use of clomiphene, a drug known as a selective oestrogen receptor
modulator or SERM. This binds to the
oestrogen receptor in the hypothalamus and instead of stimulating the receptor
it blocks its action, resulting in low levels of oestrogen. When levels of oestrogen fall this triggers
release of FSH from the anterior pituitary and this helps return the menstrual
cycle back under normal hormonal control, which should result in ovulation.