Pregnancy and embryonic development

1. Cleavage: about 24 hours after fertilisation the zygote divides by mitosis into 2 cells (blastomeres) in the upper tube, then 4 cells the next day, and so on, with no growth in size of the whole embryo.
2. The solid ball of small cells is the morula, moved down the tube by the cilia and muscle contraction. It becomes a hollow ball of cells, the blastocyst, which reaches the uterus and implants in the thick, richly supplied endometrium by the end of the first week.
3. Embryonic membranes. As the embryo grows, organogenesis and differentiation start, and two embryonic membranes form:
   • Amnion (inner): encloses the embryo in the amniotic fluid, which protects it from shocks and drying and allows free movement.
   • Chorion (outer): surrounds the amnion; has finger-like projections (chorionic villi) which grow into the endometrium; maternal and fetal capillaries come close together there and form the placenta.
4. Umbilical cord: connects the embryo with the placenta (about 70 cm long at term); rich in blood vessels; it carries digested food, vitamins, water, salts and oxygen from the placenta to the embryo, and carries excretory wastes and carbon dioxide from the embryo to the placenta.

The placenta:

1. Allows exchange between the blood of mother and embryo by diffusion (and active transport): food, water, oxygen and vitamins pass to the embryo, and wastes pass back. The two bloodstreams normally do not mix, which avoids clotting, differences of blood group, and high maternal blood pressure damaging the fetal vessels.
2. It secretes the hormones progesterone and oestrogen (from the fourth month), taking over from the corpus luteum, and relaxin.
3. Protection: it filters some bacteria and passes antibodies from mother to baby (IgG), but allows alcohol, nicotine, drugs and some viruses (e.g. rubella) to pass, which can cause serious deformities. So pregnant women must avoid smoking, alcohol, unprescribed drugs and infectious diseases.

Stages of pregnancy (nine months):

• First three months: the nervous system and heart start to develop (first month); arms and eyes differentiate; the sexes become distinct (testes at the 6th week, ovaries at the 12th week); the embryo starts to respond to stimuli. This is the most sensitive period for deformities.
• Middle three months: the heart development is complete and its beats can be heard; the skeleton ossifies; sense organs are complete; the fetus grows in size.
• Last three months: the development of the brain is complete and growth slows; other systems are completed.

Birth (labour).

• In the ninth month, the placenta begins to separate gradually; the progesterone level falls (and oxytocin and relaxin act), the attachment of the endometrium loosens.
• Labour starts with a series of uterine contractions (stimulated by oxytocin) until the fetus is expelled.
• The baby starts life with a cry that stimulates its respiratory system; the placenta separates and is expelled (afterbirth); the umbilical cord is cut from the baby's side; the baby's food now becomes the mother's milk.
• Prolactin (pituitary) stimulates milk production and oxytocin the milk ejection.
• Breast feeding gives the baby the best nutrition and emotional care and protects him from many physical and psychological diseases.
• The best age for pregnancy is from 18 to 33 years; outside the range both mother and embryo risk problems and the possibility of deformed babies increases.
• Old age of the father also increases these risks.