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Pregnancy
Description
Pregnancy is the process by which a woman carries live offspring from conception until the time when the offspring is capable of life outside the womb. It begins with conception, which is the process of fertilisation and the formation of the zygote (the fertilised egg cell after the fusion of the male and female gametes), and ends with birth, caesarean section, miscarriage or abortion. The term pregnancy is sometimes also applied to some other mammals, especially primates. For animals, however, the term gestation is usually used.
Pregnancy lasts about 40 weeks from the first day of the last menstrual cycle until birth (38 weeks from fertilisation). It is divided into three trimesters. In the first trimester there is the highest risk of spontaneous miscarriage of the foetus. A miscarriage is often the result of defects in the foetus or its parents, or of damage caused after conception.
Fertilisation
The first step of pregnancy usually begins with sexual intercourse, when the male sex cells (sperm) are introduced into the womb. Male semen contains not only sex cells but also sugars, proteins and other substances that keep the sperm viable. Human sperm usually survive in the female body for about 48 hours. Sperm have a long tail (flagellum), which they use to move; they are the only human cells with this property. These cells are haploid (cells with one set of chromosomes) and arose by meiosis from germ cells in the testicles. A single ejaculation (discharge of semen) usually contains 100 to 300 million sperm. Eggs (secondary oocytes) are haploid female reproductive cells whose role is to merge with one sperm and subsequently form a fertilised zygote. These cells are produced by meiosis in the ovaries, where they remain until they are activated and released by hormonal changes in the female menstrual cycle. Usually only one egg is released during a menstrual cycle.
During ovulation, the edge (fimbriae) at the end of the fallopian tube moves over the ovary to catch the released egg. In fertilisation, sperm usually meet the egg in the fallopian tube, which requires the sperm cells, before reaching the tube, to swim from the top of the vagina through the cervix and along the length of the womb, which is a considerable distance in proportion to the size of a sperm. After reaching the fallopian tube, the sperm swim towards the egg (orienting themselves by chemotaxis) and all try to fertilise it. Each sperm cell contains a small sac of enzymes that serve to break down the surface of the egg and thus allow the sperm to penetrate inside. This can take up to 20 minutes. When the egg merges with one sperm, its surface changes, which prevents other sperm from penetrating. The merging of the nuclei of the egg and the sperm to form a diploid cell ends the first phase of pregnancy.
Alternative methods of fertilisation, collectively referred to as assisted reproduction, are used in infertility or to fertilise women without a partner. Some factors, such as polycystic ovaries, obesity in the woman, too low a body weight in the woman or an obese man, reduce the chance of successful fertilisation.
Symptoms of pregnancy
Symptoms of pregnancy are divided by stage into early and late. The main symptom of early pregnancy is a missed regular period, morning sickness, a frequent urge to urinate, fatigue and hypersensitivity. The most common symptoms of pregnancy are listed below:
- tender, swollen breasts
- fatigue
- implantation bleeding
- nausea or vomiting
- increased sensitivity to smells and changes in taste
- a bloated abdomen
- frequent urination
- a missed period
- basal temperature stays high
- a positive pregnancy test
Development
Before implantation of the fertilised egg
At this time there is a single totipotent cell (able, by dividing, to produce every cell type in the organism) - the zygote - a genetically unique new organism. Mitotic cell division is the next process, in which each cell divides into further diploid cells. The zygote divides, producing 2 smaller cells called blastomeres, which then also divide every 20 hours. These cells gradually become smaller, until after about 4 divisions 16 individual cells are formed. This group of 16 cells is called a morula and usually leaves the fallopian tube and moves on into the womb. If it does not reach the womb, an ectopic pregnancy (outside the womb) results.
After implantation of the fertilised egg
The blastocoel is a small cavity in the centre of the cells, and the developing cells grow around it. On the surface of this cavity there is a thin layer of cells, and the zona pellucida (the membrane covering the mature egg) stays the same size as before. The cells grow ever smaller in order to fit. This new structure with a cavity in the centre and developing cells around it is called a blastocyst.
The existence of the blastocyst signals the formation of 2 types of cells: inner cells growing inside the blastocoel and cells growing on its outside. Within 24 - 48 hours the membrane of the blastocyst (zona pellucida) ruptures. The cells on the surface of the blastocyst begin to secrete an enzyme that breaks down the lining (epithelium) of the womb and creates a place for implantation.
The circulatory system of the placenta
The cells surrounding the blastocyst now break down the cells on the surface of the womb, forming small reservoirs of blood that stimulate the formation of capillaries. This is the first phase of placental growth. The inner cells of the blastocyst divide rapidly, forming 2 layers. The upper layer will become the embryo, and cells from it will be used in the amniotic cavity. At the same time the lower layer forms a small sac. (If the cells begin to develop in an abnormal position, an ectopic pregnancy can now arise).
Within a few days, the chorion tissues (the outer foetal membrane) in the forming placenta anchor the implanting part to the womb. A system of blood and blood vessels now develops at the point of the forming placenta, growing at the site of implantation. The small sac inside the blastocyst begins to produce red blood cells. Over the next 24 hours, connective tissue develops between the growing placenta and the growing foetus, which later develops into the umbilical cord.
Cell differentiation
Subsequently, a narrow line of cells appears on the surface of the embryo. Its growth shows that the foetus is about to undergo gastrulation, in which 3 layers of the foetus develop - the ectoderm (outer), the mesoderm (middle) and the endoderm (inner). The narrow lines of cells begin to form the endoderm and mesoderm. The ectoderm begins to grow rapidly as a result of chemical components produced by the mesoderm. These 3 layers form the basis of all the different types of tissue in the body.
The endoderm later forms the lining of the tongue, the digestive tract, the lungs, the stomach and several glands. The mesoderm forms the muscles, bones, lymphatic tissue, the inside of the lungs, the heart, and the reproductive and excretory systems. It also forms the basis for the spleen and for the production of blood cells. The ectoderm forms the skin, nails, hair, the cornea, the lining of the inner and outer ear, the nose, the cavity, the mouth, the rectum, the teeth, the pituitary gland, the mammary glands, the eyes and all parts of the nervous system.
About 18 days after fertilisation, the embryo has divided to form the number of tissues it will need. It is pear-shaped, with the head region larger than the tail. The embryo's nervous system is one of the first organs to grow. It begins to grow in a hollow area called the neural groove.
The blood system continues to form networks that allow blood to circulate in the embryo. Blood cells have already been formed and flow in these forming networks. Around the placenta, secondary blood vessels begin to develop to improve the supply of nutrients. Blood cells begin to form on the sac in the centre of the embryo, as do cells that begin to differentiate into blood vessels. Endocardial (inner heart) cells begin to form the muscle that will become the heart.
Within about 24 days after fertilisation, a simple S-shaped tubular heart exists and begins to pulsate. At this stage blood starts to flow through the embryo.
Month 1 (week 4)
The embryo's head and legs grow, and the eyes and ears develop. The heart is beating in the future chest and the lungs are forming.
Month 2 (week 8)
The unborn child stops being an embryo and becomes a foetus. The foetus already has all its organs. The jaw is forming, including the buds of teeth in the gums. There is electrical activity in the brain. The foetus measures about 2 cm and its heart beats about 140 to 150 times a minute.
Month 3 (week 13)
The heartbeat can be heard with a stethoscope. Fingers with soft nails have formed. Fingerprints are evident on the fingers. The foetus reacts to touch. The sex organs are developing, but determining the sex is still difficult. The foetus moves, clenches its fist and moves its head. At the end of the third month (the first trimester) the foetus is about 10 cm long and weighs about 150 g.
Month 4 (week 17)
The foetus hears the mother's voice, her heartbeat and other surrounding sounds. The amniotic fluid is renewed two to three times a day. The mother gains about a quarter of a kilogram a week. The foetus measures about 16 cm and weighs about 250 g.
Month 5 (week 21)
The mother's breasts continue to enlarge, and she usually feels the baby's first movements. The foetus is about 19 cm long and weighs 350 g. Hair and eyelashes begin to grow. Movements can be seen on an ultrasound examination and it is often possible to tell the sex.
Month 6 (week 25)
The foetus is active and turns over. It "sucks" its thumb (in reality, however, it does not yet have a developed sucking reflex). It has a real chance (about 70%) of survival in the event of premature birth. It measures over 30 cm and weighs about 900 g.
Month 7 (week 29)
The foetus grows rapidly and gains weight. It kicks, may hiccup and cry. It can distinguish between sweet and sour tastes. All five senses are working. It measures almost 40 cm and weighs about 1.8 kg.
Month 8 (week 34)
The foetus's brain has grown rapidly. In the event of premature birth, the baby has a very good chance of survival. It measures about 42 cm and weighs 2.3 kg.
Month 9 (week 38)
The birth is not far off. The baby measures from 48 to 54 cm and weighs from 2.8 to 4 kg. The diameter of its head is 9.5 to 10.5 cm.