HUMAN BIOLOGY

344 Chapter 17

Disorders: miscarriages, stillbirths,

and Birth Defects

carbohydrates, lipids, and proteins it needs. Vitamins and

minerals are a different story, however. Physicians recom-

mend that a pregnant woman take supplemental vitamins

and minerals, not only for her own benefit but also to meet

the needs of her fetus. This is particularly true for the nutri-

ent folic acid (folate), which is required for the neural tube

to develop properly. If too little folic acid is available, a birth

Miscarriage is the death and spontaneous expulsion of an

embryo or fetus before 20 weeks of gestation. It occurs in

more than 20 percent of all conceptions, usually during the

first trimester. Many factors can trigger a miscarriage (also

called spontaneous abortion), but in up to half of cases the

embryo (or the fetus) has one or more genetic disorders that

prevent normal development.

Death of a fetus after 20 weeks is called a stillbirth.

Some stillbirths result from complications during delivery,

but most occur before labor begins. The risk is greatest for

twins (or other multiples) who share a placenta and for the

fetuses of women who are diabetic, smokers, have a history

of drug abuse, or who are of African descent.

Poor maternal nutrition puts a fetus at risk

A pregnant woman must nourish her unborn child as well

as herself. In general, the same balanced diet that is good

for her should also provide her developing baby with all the

period of dividing

zygote, implantation,

and early embryo

1

CNS

heart eye

eye ear palate

brain

central nervous system

external genitals

upper limbs

lower limbs

eyes

ear

teeth

palate

ear

external genitals

teeth

limbs

heart

indicates common site of action of teratogen

not

vulnerable to

teratogens

prenatal death major structural abnormalities

age of embryo (in weeks) fetal period (in weeks) full term

2345 678916 20–36 38

functional defects and

minor structural abnormalities

heart

F i g u r e 17.19 Many factors can cause birth defects. Light blue bars indicate periods when developing organs are most sensitive to

damage from alcohol, viral infection, and so on. Dark blue bars indicate the time when major structural abnormalities due to other

causes become apparent. Numbers signify the week of development. (© Cengage Learning)

1 7. 1 0

An exposed spinal cord due to spina bifida

Biophoto Associates/Science Source

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