The fetal stage of development lasts from the end of the second month, or eighth week, until the end of the fortieth week. While all organs and organ systems are in place by the beginning of the fetal stage of development, this stage nevertheless represents an exceedingly important time. Though embryos are not viable outside their mothers, fetuses are, meaning that the fetal stage is the one that turns an underdeveloped organism incapable of independent life into a breathing, thinking, crying, functional infant.
Remainder of the First Trimester
The American Pregnancy Association notes that while a portion of the first trimester, weeks one through eight, to be precise, is given over to earlier developmental phases, weeks nine through thirteen are a part of the fetal stage of development. During this time, organs formed in the embryonic stage continue to grow and gain function. The heart, for instance, though present and beating since the fifth week, is refined into a four-chambered organ during the ninth. Developing joints increase the fetus' mobility, allowing it to move around and even begin to suck its thumb. Though the fetus is mobile, the mother won't feel its movements until well into the second trimester, because it's still quite small and there's plenty of room to maneuver without disturbing the mother's abdomen. Fingernails, fingerprints, and hair also develop in the later weeks of the first trimester, and though the bones won't finish hardening until after birth, the initially cartilaginous skeleton hardens significantly.
Second Trimester
The second trimester encompasses weeks 14 through 27 and is a time of further growth and development. In their book "What to Expect When You're Expecting," Heidi Murkoff and Sharon Mazel note that major second trimester milestones include increased movement and the ability to sense the outside world. Around week 15, the fetus' eyes become sensitive to light, though they are hidden behind sealed eyelids that won't open until the third trimester, and near the same time, the fetus begins to respond to light touches through the mother's stomach. Its heart is beating a prodigious 140 or so beats per minute, a rate that will be maintained throughout the rest of the pregnancy. By about the 22nd week, the fetus can hear and respond to sound, and will begin to learn familiar noises--including those of its parents' voices.
Third Trimester
Lasting from the beginning of the 28th week until delivery--a baby is considered full term at 40 weeks, but normal deliveries take place within two weeks in either direction of an estimated due date--the third trimester involves significant growth and weight gain. At the beginning of this trimester, most fetuses are around 2.5 pounds and about 16 inches long, according to "What to Expect When You're Expecting." At the time of deliver, infants average around 8 pounds and 18 inches in length, meaning that a major task of this period is the accumulation of extra body fat. Lung development, too, takes place during the third trimester, and babies born too early in this period can be susceptible to respiratory infection and lung collapse, notes Dr. Miriam Stoppard in her book, "Conception, Pregnancy and Birth."
References
- American Pregnancy: Fetal Development
- "What to Expect When You're Expecting"; Heidi Murkoff and Sharon Mazel; 2008
- "Conception, Pregnancy and Birth"; Miriam Stoppard, M.D.; 2008
