Page 170 - Alaska A & P Primer

28 3 Fetal Development
The fetal period lasts from the ninth week of development until birth. During this period, male and female gonads differentiate. The fetal circulatory system becomes much more specialized and efficient than its embryonic counterpart. It includes three shunts—the duc- tus venosus, the foramen ovale, and the ductus arteriosus—that enable it to bypass the semifunctional liver and pulmonary circuit until after childbirth.
The brain continues to grow and its structures differentiate. Facial features develop, the body elongates, and the skeleton ossifies. In the womb, the developing fetus moves, blinks, practices sucking, and circulates amniotic fluid. The fetus grows from an embryo measur- ing approximately 3.3 cm (1.3 in) and weighing 7 g (0.25 oz) to an infant measuring ap- proximately 51 cm (20 in) and weighing an average of approximately 3.4 kg (7.5 lbs). Em- bryonic organ structures that were primitive and nonfunctional develop to the point that the newborn can survive in the outside world.
Sexual differentiation does not begin until the fetal period, during weeks 9–12. Embryonic males and females, though genetically distinguishable, are morphologically identical.
During weeks 9–12 of fetal development, the brain continues to expand, the body elon- gates, and ossification continues. Fetal movements are frequent during this period, but are jerky and not well-controlled. The bone marrow begins to take over the process of erythro- cyte production—a task that the liver performed during the embryonic period.
Weeks 13–16 are marked by sensory organ development. The eyes move closer together; blinking motions begin, although the eyes remain sealed shut.
During approximately weeks 16–20, as the fetus grows and limb movements become more powerful, the mother may begin to feel quickening, or fetal movements. However, space restrictions limit these movements and typically force the growing fetus into the “fetal posi- tion,” with the arms crossed and the legs bent at the knees.
Developmental weeks 21–30 are characterized by rapid weight gain, which is important for maintaining a stable body temperature after birth. The bone marrow completely takes over erythrocyte synthesis, and the axons of the spinal cord begin to be myelinated, or coated in the electrically insulating glial cell sheaths that are necessary for efficient nerv- ous system functioning. The fetus continues to lay down subcutaneous fat from week 31 until birth.
28 4 Maternal Changes During Pregnancy, Labor and Birth
A full-term pregnancy lasts approximately 270 days (approximately 38.5 weeks) from con- ception to birth. Because it is easier to remember the first day of the last menstrual period (LMP) than to estimate the date of conception, obstetricians set the due date as 284 days (approximately 40.5 weeks) from the LMP. This assumes that conception occurred on day 14 of the woman’s cycle, which is usually a good approximation. The 40 weeks of an aver- age pregnancy are usually discussed in terms of three trimesters, each approximately 13 weeks. During the second and third trimesters, the pre-pregnancy uterus—about the size of a fist—grows dramatically to contain the fetus, causing a number of anatomical changes in the mother.
As the woman’s body adapts to pregnancy, characteristic physiologic changes occur. These changes can sometimes prompt symptoms often referred to collectively as the common discomforts of pregnancy. Nausea and vomiting, sometimes triggered by an increased sen- sitivity to odors, are common during the first few weeks to months of pregnancy. This phe- nomenon is often referred to as “morning sickness,” although the nausea may persist all
28.3 OBJECTIVES
1. Describe the fetal circulatory system and explain the role of the shunts
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28.4 OBJECTIVES
1. Identify and describe each of the three stages of childbirth