Fertilization and the creation of twins


Fertilisation is the fusion of a man’s sperm with a woman’s egg. It can happen when sperm enters a woman’s vagina at a time when she is fertile. A woman is fertile during the part of her monthly menstrual called ovulation. Ovulation happens about two weeks after the start of a woman’s last menstrual period. During ovulation, one of a woman’s two ovaries releases an egg. The released egg travels into the fallopian tube. If it meets a sperm there, the two can fuse. In order for a sperm to make it to the fallopian tube, it needs to swim from the woman’s reproductive tracts through the cervix into the fallopian tube. This can take from few hours up to several days. When the egg and the sperm fuse, the egg becomes fertilised. This means that the sperm releases the DNA it carries into the egg. The man and the woman each contribute half of the DNA of the baby. If the egg did not become fertilised, the woman experiences her period and her uterus becomes ready for another menstrual cycle. If it does become fertilised, the egg becomes attached to the lining of the uterus. Once the egg becomes fertilised, it starts to divide and replicate. One cell becomes two. Two cells become four. Four cells become eight and so on until the billions and billions of cells needed to make a baby are formed. As more and more cells are made, they start to specialise. Some cells become part of the baby’s heart. Some become part of the liver. Some become fingernails, and some become brain cells. After about 9 months, a fully formed baby is born.


Most of the time, one sperm meets one egg and the woman becomes pregnant with one baby. Sometimes a woman becomes pregnant with twins. Twins are two children born at the same time to the same mother. There are two types of twins: identical and non-identical twins. Identical twins happen when one fertilised embryo splits into two. Because identical twins come from the same fertilised egg, they have the same genes. This means that they always have the same sex. They are either both boys or both girls. Identical twins also look very similar to each other. Non-identical twins happen when two eggs are fertilised by to sperms at the same time. This can happen when a woman’s uterus releases two eggs during a menstrual cycle instead of one. Non-identical twins share the same amount of DNA with each other as normal siblings. They therefore look no more alike than any other brothers or sisters. Non-identical twins can be either both boys, both girls, or a boy and a girl.




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