The development of a fertilized ovum into a newborn infant takes about 266 days from conception to birth. During most of pregnancy, the fetus grows in a fluid-filled environment contained within a double-layered sac called the fetal membranes. Stages of development can be divided into three trimesters of pregnancy, with accelerated growth in the first trimester encompassing day-to-day embryonic changes, leading to fetal growth, which is typically measured in weeks. Mothers may not realize their unborn baby has hair, fingernails and the ability to hear her voice long before the end of pregnancy.
First Trimester
Conception occurs approximately 14 days after the first day of the woman's last menstrual period, if her cycle normally lasts 28 days. The pre-embryonic period begins with fertilization, which takes about 24 hours and usually occurs near the mouth of the fallopian tube. The placenta begins to function by the end of the third week following conception. According to the Mayo Clinic, an embryo's organ systems develop during the fourth to eighth week, and exposure during this time to drugs, chemicals, viruses, radiation and other known teratogens do the most damage to differentiating tissues. The end of the eighth week marks the fetal period, when the developing baby begins to look human and has a heartbeat. Growth of the body catches up somewhat to growth of the head. By the end of 12 weeks, the crown-to-rump length has more than doubled and genitals take on a distinctly male or female appearance. At the end of the first trimester of pregnancy, the fetus weighs about 3/4 oz. and is approximately 3 inches long.
Second Trimester
Although the organ systems have formed, much maturing is necessary before the fetus can survive outside the mother's body. The limbs grow and toenails appear. Movement can be felt by the mother at about 15 weeks gestation. Bones and fatty tissues appear, and lanolin-like vernix protects the skin. A fine, downy hair covers the body. By 20 weeks the baby has eyebrows, can hear the mother's voice and will be startled by loud sounds. The fetus can also dream. Babies born prematurely near 24 weeks may survive if given intensive care but could suffer long-term disabilities. At the end of the second trimester, the fetus weighs approximately 1 1/2 lbs. and is just over 11 inches long.
Third Trimester
The baby is now preparing for life outside the womb. The brain, lungs and nervous system are rapidly maturing as the fetus gains weight. The baby has reflexes that will help him find the breast, suck and breathe. According to Anne Frye, author of "Holistic Midwifery," 95 percent of babies assume a head-down position by 34 weeks. Birth is likely to occur between 38 and 42 weeks. The average baby weighs a plump 7 1/2 lbs and is about 20 inches from crown to heels.
References
- Mayo Clinic: Fetal Development
- "Holistic Midwifery," Anne Frye, 1998
