The average length of time for prenatal development is 38 to 40 weeks after conception. During this time, a baby will develop from a nearly invisible group of cells to a full-term infant. Prenatal development is divided into three stages: the germinal stage, the embryonic stage, and the fetal stage. Occasionally, there are physiological or environmental factors that can hinder normal prenatal development.
Germinal Stage
The germinal stage marks the very beginning of a pregnancy. During this stage, a sperm and egg combine, and the newly fertilized egg, known as a zygote when it is just one cell, will travel to the uterus where it will implant and undergo cell division. During cell division, the cells will develop into a blastocyst, which is made up of three layers: the ectoderm, endoderm, and mesoderm. These layers will become the skin and nervous system, the respiratory and digestive system, and the musculo-skeletal system, respectively. The end of this stage is marked by implantation of the blastocyst on the uterine wall.
Embryonic Stage
Three weeks after conception, the developing cells, which began as a single-celled zygote, are considered an embryo. The embryonic stage of development will continue from week three to week nine. During this stage, the embryo will develop the basic body systems that are necessary to sustain life. The embryonic stage is a crucial period in pregnancy, as it involves rapid development of important organs such as the heart, lungs, and brain. One of the most important developmental events that will take place during this stage is the closing of the neural tube, which will later form the spinal cord.
Fetal Stage
Eleven weeks after conception, the nearly 2-inch embryo officially becomes a fetus. This is the final stage of pregnancy, and will last until the baby is born some time between 38 and 40 weeks after conception, making it the longest of the prenatal stages. During the fetal stage, the organs and structures formed during the embryonic stage will continue to develop and become bigger and stronger. The genitals are well formed at week 11, and may be visible on an ultrasound scan. The fetus will begin this stage measuring only a couple of inches in length, and will leave this stage measuring in at 19 to 21 inches.
Factors That Can Alter Normal Prenatal Development
While many pregnancies develop at a predictable rate, there are some factors that can cause an embryo or fetus to develop abnormally. Certain congenital abnormalities, for example, can cause anywhere from minor to major delays in development. These types of abnormalities can be caused by genetics or by a random malfunction of the prenatal developmental process. Environmental factors such as maternal alcohol or drug use can drastically hinder or stop an embryo or fetus from developing. Deficiency in certain vitamins can also increase the risk of developmental abnormalities. Folic acid deficiency, for example, has been linked to an increased risk of neural tube defects. To promote normal prenatal development, it is important to eat a well-balanced diet and receive regular prenatal medical care.
