Pregnancy is the period from conception to birth. The ovum (egg) gets fertilized with the sperm at the lateral part of fallopian tube named ampulla. This fertilized ovum (egg) is implanted in the uterine wall and gradually grows into fetus. Pregnancy usually lasts for 40 weeks, begins from the day of last menstrual period and ends with the complete expulsion of the products of conception. The entire period is divided into 3 trimesters.
At the end of the first month, the embryo is about one fourth of an inch long. Amniotic sac, a water-tight sac surrounds the growing embryo. The embryo receives nutrients and eliminates waste through the umbilical cord and placenta. By the end of the first month, the liver and digestive system begin to develop. Blood cells develop and circulation begins. Heart beats at a rate of 65 per minute.
Fetus is 1 inch long. Nervous system starts developing. A complete cartilaginous skeleton will develop into bones at the end of the month. Facial features continue to develop. Arms, legs and all of the major organs begin to appear. Fetal heart beat can usually be detected by around 6 week.
The fetus has now grown to 4 in (10 cm) and weighs about 28 g. The baby will have human appearance. The arms, hands, fingers, feet, and toes are fully formed. Now the major blood vessels, few of the internal organs and extremities are well developed. The reproductive organs begin to develop but at this stage sex determination using ultrasound is impossible. Chances are less for miscarriage or abortion after three months. With this the first trimester ends.
Now the baby is 6 inches long. A fetal Doppler can reveal the fetal heart beat. The fingers and toes are well-defined. Eyelids, eyebrows, eyelashes, nails, and hair are formed. Teeth and bones become denser. The fetus begins to kick and swallow, and even sucks the thumb, yawn and stretch. Sex determination by ultrasound is possible.
Fetus weighs around 454 g and is 8-12 inch long. Internal organs continue to grow at a faster rate. At this point, the mother may feel her baby moving inside her womb. This first movement identified by the mother is termed as quickening. It occurs around 20th week. Hair begins to grow on baby’s head. Lanugo, soft fine hair, covers the shoulders, back, and buttocks. The skin is covered with a whitish cheesy substance called vernix caseosa and it protects fetal skin from the constant exposure to the amniotic fluid.
By the end of the second trimester, fetus is about 12 inches long and weighs around 700 gm. Even though the lungs are not completely formed, the baby delivered prematurely after 23rd week can survive with intensive care. The fetus will grow very fast during this month. The eyelids begin to separate and the eyes open. Baby responds to sounds by moving or increasing the pulse. Baby floats in the amniotic fluid and may have different fetal presentations.
Fetus has grown to about 1.3 kg in weight and 14 inches in length by the beginning of last trimester. Organs mature and develop reserves of body fat. The auditory system is completely developed. The amniotic fluid begins to diminish. Better chances for survival even if born prematurely.
Baby has a weight of about 1.8-2.3 kg and measures around 40-45 cm. The baby occupies most of the space in the uterus. Growth continues but at a slower rate. The presentation of the baby come backs to normal, with the head end at the lower pole of the uterus. This is called a cephalic presentation. The fetal movements are almost evident. Most internal systems are well developed, but the lungs may still be immature.
Full term baby weighs about 2.6 to 3.4 gm and is about 20 long. Amount of amniotic fluid will decrease. Descend of the fetal presenting part favors the onset of labor, which may begin any time between the 37th and 42nd week of gestation. Baby's reflexes are well coordinated so he or she can blink, close the eyes, turn the head, grasp firmly, and respond to sounds, light, and touch. The presenting part sinks down into the pelvic cavity. Once the labor is initiated the fetus is passed out to the external world through the birth canal.