27.9A: Fetal Development
At the end of the 10th week of gestation, the fetal period begins.
- Outline the progression of fetal development from 11 weeks to 40 weeks
Key Points
- Although fetuses can be adversely affect by environmental toxicants, they are less sensitive to them than embryos because the precursors for most organs are already formed.
- At the start of the fetal stage, the fetus is typically about 30 millimeters in length from crown to rump, and weighs about eight grams. The head makes up nearly half of the fetus’ size.
- The precursors of all the major organs are created by the time the fetal period begins. Therefore, the fetal period is described both by organ and a list of the changes by weeks of gestational age.
Key Terms
- meconium : The earliest stool of a mammalian infant. Unlike later feces, meconium is composed of materials ingested during the time the infant spends in the uterus (intestinal epithelial cells, lanugo, mucus, amniotic fluid, bile, and water).
- gestational age : This relates to the age of an embryo or fetus (or newborn infant). In human obstetrics, this age is often defined as the time elapsed since 14 days prior to fertilization; this is approximately the duration since the woman’s last menstrual period began.
- lanugo : Soft down or fine hair that covers the human fetus.
In humans, the fetal stage of prenatal development starts at the beginning of the 11th week in gestational age, which is the ninth week after fertilization. Since the precursors of all the major organs are created by this time, the fetal period is described both by organ and a list of changes by weeks of gestational age.
All major structures are already formed in the fetus, but they continue to grow and develop. Therefore, the fetus is not as sensitive to damage from environmental exposures as the embryo, though toxic exposures often cause physiological abnormalities, growth retardation, or minor congenital malformations.
At the start of the fetal stage, the fetus is typically about 30 millimeters in length from crown to rump and weighs about eight grams. The head makes up nearly half of the fetus’ size. The four-chamber heart is finishing development, and the embryonic tail goes away. The breathing-like movement of the fetus is necessary for stimulation of lung development, rather than for obtaining oxygen.
- Week 10: Finger nails and hair start to grow. The heart, hands, feet, brain, and other organs are present, but are only at the beginning of development and have minimal operation.
- Week 11: Nearly all structures and organs are formed. Fingers and toes are separated and the genitals begin to take on the proper gender characteristics.
- Week 12: The digestive system and liver function. The pancreas makes insulin.
- Week 13: The fetus begins to get its nourishment from the placenta and the veins and organs are visible through the skin.
- Week 14: The kidneys produce urine, and the liver makes bile. In boys, the prostate gland develops, while in girls the ovaries move from the abdomen to the pelvis.
- Weeks 15 to 16: The heart pumps out 25 quarts of blood a day and the fetal structures are looking more normal.
- Week 17: The fetus start to move its joints and the retina becomes sensitive to light. It weighs about five ounces.
- Week 18: The fetus starts to hear and is startled by noise. Its skin is starting to grow a protective, wax-like layer and tiny air spaces begin to form in the lungs and the vocal cords.
- Week 19: The brain is designating areas for the five senses.
- Week 20: The fetus weighs about 10 ½ oz, swallows more, and produces meconium.
- Week 21: The eyebrows and lids are present, and for the female fetus, the vagina begins to form.
- Week 22: Tiny tooth buds beneath the gums develops. The eyes form, but the irises lack pigment.
- Week 23: The fetus weighs over a pound. It can feel movements and hear sounds. Blood vessels in his/her lungs are developing to prepare for breathing.
- Week 24: The fetus is almost a foot long. The lungs are developing branches of the respiratory tree as well as cells that produce surfactant, a substance that will help the air sacs inflate once born.
- Week 25: The amount of body fat rapidly increases. The lungs are not fully mature. The bones are fully developed, but are still soft and pliable. Thalamic brain connections form that mediate sensory input. Iron, calcium, and phosphorus become more abundant. The fingernails reach the end of the fingertips. The lanugo, or fine hair, begins to disappear, until it is gone (except on the upper arms and shoulders). Small breast buds are present on both sexes. Head hair becomes coarse and thicker.
Birth is imminent and occurs around the 40th week. The fetus is considered full-term between weeks 37 and 40, which means that the fetus is considered sufficiently developed for life outside the uterus.