How Does the Development of an Unborn Baby Progress?

Regardless of an individual's personal views, the life of a new being begins when the genetic information from the father's sperm fuses with the genetic information of the mother's egg. This moment is called conception. The newly fertilized egg, or zygote, will implant in the uterine lining of the mother. Here it begins to form a network of blood vessels that it will use to nourish and support itself until the time of birth approximately nine months later.

Conception to Implantation

• Shortly after fertilization, the newly formed zygote begins to divide, doubling its number of cells with each division. During this time, the ball of cells, now a morula, navigates through the woman's fallopian tubes on its way to the uterus. This journey lasts about six days. By eight days past fertilization, the newly formed embryo begins to burrow into the lining of the uterus. Once fused to the uterine lining, the placenta begins to form. The placenta is rich in blood vessels used to carry nutrients to and take waste away from the developing baby. The joining of the circulatory systems of mother and child leads to hormonal changes in the mother, causing pregnancy's earliest symptoms.

The First Trimester

• Pregnancy is divided into three trimesters, each three months long. By the time the embryo is 2 weeks old, it is oblong in shape and has begun to form a brain. Changes begin to take place rapidly, and the most crucial period of development occurs. Sometime between the third and fourth week of life the heart begins to beat. Between the fourth and sixth week of life, the vertebrae and ribs begin to form in the torso and jaw, face and neck begin to take shape. The nervous system begins to relay signals between the brain and the rest of the body by the seventh week. By this time, the hands resemble paddles and the eyes are forming. The embryo doubles in size in the seventh week, growing from about 5 mm in length to 10 mm. By the time the embryo is 8 weeks old, many vital organs are formed and functioning and the baby is constantly in motion. Week 10 marks the change from embryo to fetus. The fingers and toes begin to take on more definition and the baby measures close to 3 cm in length. The most vulnerable period of development is now over, and the risk of miscarriage drops significantly.

The Second Trimester

• The fetus continues to grow rapidly, and is about the size of an orange at the beginning of the second trimester. The developing skeleton, made of cartilage, begins to turn into bone. The external genitals start to form during this time as well. The hair, which started sprouting out of the skin in week 10, begins to take on pigment and thickens on the eyebrows and the scalp. When the embryo is 15 to 16 weeks old, it is about 7 inches long and has genitals developed enough to differentiate by ultrasound--that is, if it cooperates. On some ultrasounds taken around this time, the fetus is seen sucking its thumb. By the end of the second trimester, the fetus is almost 12 inches long, and is able to hear its mother's voice, her heart and her digestive system.

The Third Trimester

• At the beginning of the third trimester, the fetus can open its eyes for the first time. It is just more than 1 foot in length and weighs close to 2 lbs. Most of the rest of the pregnancy is dedicated to storing fat, putting on weight and practicing breathing. Sometimes, breathing exercises cause the baby to get hiccups. When the baby is in its 34th week, it weighs about 5 lbs. and is 13.5 inches long. By the time the baby is in its 38th week, it is time to be born.

Getting Ready For Birth

• During the last month of pregnancy, the womb becomes especially crowded as the baby gains a full 2 lbs. or so of its birth weight. In most cases, as the end of the pregnancy nears the baby will turn itself upside down and be born head first. As the birth date nears, the baby drops lower into the mother's pelvis. This increases the pressure on her cervix, helping it to thin out (efface) and open up (dilate) so that the baby can make its way through the birth canal.