Spermatoza remain viable in the female genital tract from 24 to 72 hours. For fertilization, sperms are introduced into the female body. One sperm fuses with the ovum in the fallopian tube.
If the ovum happens to meet a sperm, the two unite to form a zygote. 13-14 days after onset of menstruation are most favourable for conception (pregnancy). The zygote immediately begins to divide and passes down the fallopian tube to the uterus and fixes itself to the wall of the uterus. This fixing of the embryo in the wall of the uterus is called implantation and the female is said to be pregnant. Implantation takes place about a week after fertilization.
Placenta is an association between maternal and foetal tissue meant for some extermely important physiological exchange. The developing embryo is attached to the uterus by a tissue called placenta.
Umbilical cord is a tough structure that serves as the blood vascular connection between the foetus (developing embryo) and uterine wall. From the first few weeks of development, the embryo is enclosed in a sac called amnion which is filled with amniotic fluid. Amniotic fluid acts as a shock-absorber and helps to protect the embryo from damage.
Placenta serves as a tissue through which oxygen and food are supplied from the maternal blood to the foetus. It also transports carbon dioxide and excretory waste from the foetal blood to the maternal blood.
Placenta is permeable to respiratory gases, nutrients and antibodies. The membrane prevents harmful material from reaching the embryo. It does not allow the passage of germs from the mother to the foetus.
However, if the mother is already infected with HIV, then HIV can pass through the blood to the embryo. Placenta produces the hormone progesterone. Egg-formation (ovulation) and menstruation also stop as pregnancy continues. However, these are resumed after child birth.
The hormone Oxytocin from posterior pituitary is responsible for uterine contractions for child birth.
The secretion of milk from the mammary glands is called lactation and the period during which the mammary gland secretes milk is called lactation period. The first seeretion that comes out from the mammary glands of the mother, just after child birth, is called colostrum. It is rich in nutrients, fats and proteins. Colostrum also contains antibodies (Immunoglobulin A-IgA) that provide passive immunity to the new born infant.
The synthesis of milk from the mammary glands is stimulated by the hormone prolactin which is secreted by the anterior lobe of the pituitary gland. Another hormone called oxytocin secreted by the posterior lobe of pituitary gland stimulates the release of the milk from the mammary glands.
Usually, only one ovum is released by an ovary in every reproductive cycle. If this ovum receives sperm and gets fertilized, one baby is born to the mother. But sometimes two eggs may be released and fertilized by two different sperms. Such siblings are called fraternal twins who may be brother and sister, or brother-brother, or sister-sister.
But in certain cases, only one egg is released and gets fertilized. After this, it divides into two, and the two cells then separate and start developing independently into two separate individuals. They are identical in all respects and called identical twins. They are always of the same sex.
The twins produced from one egg which fail to separate are called Siamese twins. The first case of Siamese twins was of twin boys born to a Chinese mother in Siam, (now Thailand) in 1811. These were joined at the thoracic region. These twins lived up to an age of 65 years.
The Siamese twins can sometimes be surgically separated. However, it depends upon the extent of their joining.