THE PROCESS OF CONCEPTION
Conception, or fertilization, is the fusing of an ovum (egg) and a sperm to form a new human life. This can only happen at a certain stage of a woman’s menstrual cycle, and if certain other conditions are fulfilled also. It is useful to understand theof so that we can then see how the can be interrupted.
Menstrual cycles vary a great deal from woman to woman, but there are certain factors that all normal menstrual cycles have in common. The first day of a menstrual cycle is the first day of a woman’s period (bleeding from the vagina). This bleeding is the uterus shedding its thickened lining. During the next two weeks or so the brain’s hypothalamus will stimulate the pituitary gland to produce. One of these, the follicle stimulating hormone (FsH) stimulates the development of several follicles (egg-cases) in the ovary. The other, the luteinising hormone (LH), makes the follicles produce oestrogen. Oestrogen in turn stimulates the uterus to start building up its lining to prepare to nourish a baby.
In the middle of the woman’s cycle (around day 14 in a ‘textbook 28-day cycle), the level of LH reaches a very high peak, which stimulates the dominant follicle to release an egg; this is known as ovulation. The ruptured follicle is now called a corpus luteum (‘yellow body’) and begins producing progesterone as well as oestrogen. Around the time of ovulation the basal (resting) body temperature drops slightly and then rises significantly immediately afterwards, and maintains a higher level for some time. The cervix (en trance to the uterus) becomes soft, the cervical canal opens wider, and the cervical mucus becomes clear and slippery so that sperm can penetrate it.
If pregnancy does not occur, the corpus lute urn degenerates after about ten days; when this happens oestrogen and progesterone levels fall rapidly. The hormone levels are then not high enough to maintain the thickened endometrium (lining of the uterus), and this is sloughed off as menstruation. Then the whole process begins again. If pregnancy does occur, the corpus luteum is preserved by the presence of a further hormone called human chorionic gonadotrophin (Hco); this is produced by the placenta. Between six and eight weeks into pregnancy the placenta is capable of producing sufficient oestrogen and progesterone to maintain the pregnancy, and the corpus luteum slowly degenerates as it is no longer needed.
Fertilization usually occurs in a fallopian tube. When the egg is released by the ovary the fimbria (finger-like projections at the ends of the tubes) sweep the egg into the entrance of the tube. Minute hairs (cilia) push the egg slowly down the tube towards the uterus. The full journey takes about three days, but the egg is only ripe for fertilization for about twenty-four hours, so the sperm have to travel all the way from the vagina up to the outer end of the fallopian tube. This is a hazardous journey, and sperm need to be tough and healthy to survive it. A man’s normal ejaculate contains several hundred million sperm, but only a few hundred will make it anywhere near the egg.
Sperm are produced in the man’s body by the action of hormones on the testes. Once the sperm are made they are stored for a while; the whole process from the beginning of manufacture until ejaculation is about three months. Just before orgasm the sperm are diluted with various lubricants and the resulting liquid, the semen, is ejaculated from the tip of the penis during intercourse. This means that the semen forms a pool around the cervix at the top of the vagina.
During most phases of the menstrual cycle the cervical mucus is hostile and impenetrable to sperm, but around the time of ovulation it becomes thin and slippery; this makes it into an ideal environment for the sperm to swim through. In addition the cervical canal widens, which helps the sperm to penetrate into the uterus. Chemical conditions in the cervical canal help to kill off weak or damaged sperm, and also help prepare the healthy sperm for fertilization. This is the process known as capacitation, and is vital for; sperm which have not been capacitated cannot fertilize an egg.
The tails of the sperm lash them forward in a swimming motion into the uterus; from the uterus they will then enter the fallopian tubes. (Of course, roughly half the sperm will be going on a wild goose chase up the wrong fallopian tube!) By this stage many of the sperm will have died off, and many others will do so during the journey up the fallopian tube. Only the very strongest will reach the egg, and of those only the first will be able to fertilize it.
Stages in conception:
1 Sperm are deposited near the cervix.
2 Sperm swim through the cervical canal and undergo capacitation.
3 An egg is released from the ovary.
4 Sperm swim up the fallopian tubes.
5 Sperm and ovum meet high up in one fallopian tube and fuse.
6 The fertilized ovum travels down the fallopian tube and implants in the endometrium.
The first sperm penetrates the outside of the egg, and the genetic material from sperm and egg fuses to produce a new, unique human life. The sex of the child is determined by whether the sperm was carrying an x (female) or a Y (male) chromosome. As soon as fertilization takes place with one sperm, chemical changes rapidly take place which prevent any other sperm from joining in.
The fertilized egg cell (sometimes called a blastocyst or zygote) soon begins to divide; this happens about once every twenty-four hours, so that it is two, then four, then eight cells and so on. While this is happening it is continuing its journey down the fallopian tube to the uterus. Meanwhile the uterus is receiving signals that fertilization has taken place, and completes its preparations as a hospitable environment. When the fertilized egg arrives in the uterus it implants itself in the enriched endometrium (uterus lining); this is the process known as implantation, or nidation. If all is well, the fertilized egg continues to develop into a full-grown baby, nourished via the placenta for around nine months until it is ready to be born.
Interrupting The Process Of Conception
There are five possible stages at which the process of conception and pregnancy can be interrupted:
1 There is no egg available to be fertilized.
2 No sperm are introduced into the woman’s reproductive system.
3 The sperm and the egg are prevented from meeting.
4 Implantation of the fertilized egg cannot take place.
5 The pregnancy is aborted.
The various methods of birth control are planned to work at the following stages: Stage I: combined pill; mini-pill (in part); breastfeeding; rhythm methods; long-acting hormonal methods (in part).
Stage 2: vasectomy; sheath; withdrawal; male pill; vaccination against sperm.
Stage 3: Caps; mini-pill (in part); long-acting hormonal methods (in part); sponges; douching; spermicide; female sterilization; intra-cervical devices; vaginal ring (in part.)
Stage 4: IUDS; mini-pill (in part); long-acting hormonal methods (in part); vaccination against pregnancy; post-coital IUD; post-coital pill; vaginal ring (in part).
Stage 5: abortion; menstrual extraction.