From conception to birth, a typical intrauterine pregnancy is divided into three stages or trimesters. It averages to about 40 weeks, but can be anywhere from 38 to 42 weeks.
Conception usually happens about two weeks after the first day of the last menstrual period. An egg is released from the ovaries, which passes into the fallopian tube. If this egg is fertilized by a sperm within twenty-four hours of being released, then conception occurs. The fertilized egg is called a zygote. Within twenty-four hours of being fertilized the zygote begins dividing into many cells and forms a blastocyst. It moves through the fallopian tube and reaches the uterus by the third or fourth day of conception. The lining of the uterus has thickened by then and is ready for the blastocyst to implant itself. The blastocyst attaches itself to the lining of the uterus. This is called implantation. The outer cells of the blastocyst form the placenta and the inner cells form the embryo.
When the implantation of the blastocyst in the uterus happens successfully an intrauterine pregnancy is established.
Sometimes, the ovary may release two eggs both of which may be fertilized. If both the fertilized eggs implant in the uterus, then this results in the birth of fraternal twins. At times, a fertilized egg may split into two in the early stages and continue as two distinct eggs. This results in the birth of identical twins.
The fetus rapidly grows and the progress can be followed week to week. By the end of the third month, it has a beating heart, bones, fingernails, skin, kidneys and sweat glands. The external sex organs also begin to appear by now. The fetus also makes spontaneous movements but these cannot be felt by the mother.
Nausea and morning sickness may be present at this time for the mother. The breasts grow and change. Women who are prone to acne may experience outbreaks. Women do not usually gain much weight during the first three months of pregnancy.
In the second trimester, the skin of the fetus is covered by a greasy material known as vernix caseosa which protects the skin. The uterus has formed if the foetus is female. The eyelids are fused together. Almost all the organs of the foetus develop fully in the second trimester. The bone marrow starts making blood cells and taste buds also begin to form. The body is covered by fine hair known as lanugo.
Movement is felt by the mother for the first time in the second trimester. By now nausea, if present earlier will reduce and may disappear altogether. Breasts may start producing colostrum. Symptoms such as bleeding gums, nosebleeds may appear due to increased blood flow in the body. Shortness of breath and digestive problems such as heartburn and constipation may appear.
During the third trimester, the foetus has developed almost all vital organs. It now grows in size and weight. The lanugo or hair on the body has fallen off and the pupils in the eye can constrict and dilate. It has a firm grip and skin is no longer wrinkled.
As the foetus grows it puts more strain on the mother’s body. Common pregnancy symptoms such as fatigue, troubled sleep, frequent urination, shortness of breath, varicose veins, and stretch marks continue to the end of pregnancy. If the foetus moves down to the lower part of the uterus, then this may relieve constipation and heartburn that are common earlier in pregnancy. But some foetuses do not move down until the very end of pregnancy. The baby may be born anytime between the 38th to 42nd week, but the 40th week is the usual time.