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Pregnancy occurs as a result of the fertilization of an ovum by sperm. With fertilization cell division begins, and the fertilized egg develops into a mass of cells called the morula that move through the uterine tube into the uterus. The morula continues to divide until it becomes a hollow clump of about a hundred cells called the blastocyst.

About seven or eight days after fertilization, the blastocyst settles on the wall of the uterus. Some of the cells covering the blastocyst, known as the trophoblast, begin to eat into the lining of the uterus and grow into cords that anchor the blastocyst to the wall of the uterus. The trophoblast eventually develops into the placenta.

The blastocyst is composed of two layers, the upper, ectoderm layer, and the lower, endoderm layer. An amniotic cavity and yolk sac soon appear in the blastocyst. The amnion lines the chorion, the outermost envelope that furnishes a protective and nutritive covering for the zygote, as the fertilized egg is now called.

The embryonic disk, a flat area in the cleaved ovum in which the first traces of the embryo are seen, is suspended from the chorion and is composed of three germ layers, the ectoderm, mesoderm, and endoderm. All organs of the embryo are developed from these three germ layers.

From the ectoderm the nervous system, sense organs, and epidermis, among others, are developed. The circulatory, excretory, skeletal, muscular, and reproductive systems are developed from the mesoderm. And the endoderm produces the respiratory and digestive systems, along with their linings.

Pregnancy lasts for about nine months and can be divided into approximately equal parts called trimesters. The first trimester is the period in which all the various different parts of the baby are formed. During the second and third trimesters the organs needed for the baby to survive in the outside world develop and mature, and the fetus continue to grow in size and weight.

The first sign of pregnancy is usually a missed period as the menstrual cycle is interrupted by the fertilization and implantation of the embryo cell in the uterus. Other signs of early pregnancy are nausea, more frequent urination, and a sense of fullness or tenderness in the breasts, especially around the nipples.

Nausea usually occurs early in the day and then subsides, hence the phrase "morning sickness", but some women can suffer from actual vomiting. Nausea and vomiting normally cease after the first three months of pregnancy. If you suspect you are pregnant you should see your doctor for confirmation.

There are also several home pregnancy test kits available from your pharmacy. Although positive results from these test kits are usually accurate, false negative results can occur. A doctor will probably want to give you a physical examination, in addition to performing a urine test or a sensitive blood test, to confirm pregnancy.

There are various signs a doctor looks for in the examination: a pregnant woman's breast tissue is firm and may be more tender than usual, and an internal examination will detect a change in the shape and size of the uterus and changes in the vaginal lining and cervix. In a pregnant woman the lining of the vagina becomes blue as a result of the increase in blood flowing to it, and the cervix softens so that it is possible to feel the rest of the uterus clearly through it.

The uterus softens as well and becomes larger and rounder than usual. From the doctor's internal examination it is possible to estimate how many weeks pregnant a woman is. During the first trimester the growth of the uterus is rapid and feeling the uterus at this stage is done more accurately than later in pregnancy. A pregnancy lasts for approximately 267 days.

As the date of conception is not always known it is usual to calculate the date of delivery from the start of the last menstrual period. When a mother fails to note the date of her last period or had previously been on contraceptives, a doctor may want an ultrasound examination which gives remarkably accurate information on the age of the fetus. Even if the exact date of conception is known, a doctor will frequently give a range of dates covering a 2-3 week period for the estimated date of birth.


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