HUMAN BIOLOGY

346 Chapter 17

amniocentesis Testing fetal
cells in a sample of amniotic
fluid for evidence of birth
defects.

chorionic villus sampling
Testing of fetal cells removed
from chorionic villi for birth
defects.

preimplantation diagnosis
Testing for birth defects
in an early embryo that
was conceived by in vitro
fertilization.

a growing number of technologies

now enable us to detect more than

one hundred genetic disorders

before a child is born.

Chorionic villus sampling (CvS)

uses tissue from the chorionic villi

of the placenta. CvS can be used

as early as the eighth week of

pregnancy, but it is tricky. Using

ultrasound, the physician guides a

tube through the vagina, past the

cervix, and along the uterine wall,

then removes a small sample of

chorionic villus cells by suction. results are available

within days.

Amniocentesis is performed

during the fourteenth to sixteenth

weeks of pregnancy. it samples the

amniotic fluid that surrounds the

fetus (Figure 17.21). the thin needle

of a syringe is inserted through

the mother’s abdominal wall, into

the amnion. the physician must

take care that the needle doesn’t

puncture the fetus and that no infection occurs. amniotic

fluid contains sloughed fetal cells, and as the syringe

withdraws fluid, some of those cells are included. they

are then cultured and tested for genetic abnormalities.

methods of embryo screening also are available. in

preimplantation diagnosis, an embryo conceived by

in vitro fertilization (Section 16.8) is analyzed for genetic

prenatal Diagnosis: Detecting Birth Defects

defects using recombinant Dna technology. the testing

occurs at the eight-cell stage (left), which by one view is

a pre-pregnancy stage. like unfertilized eggs discarded

during monthly menstruation, the ball is not implanted

in the uterus. its cells all have the same genes and are

not yet committed to giving rise to specialized cells of

a heart, lungs, or other organs. Doctors take one of the

undifferentiated cells and analyze its genes for suspected

disorders. if the cell has no detectable genetic defects,

the ball is inserted into the uterus. embryo screening is

designed to help parents who are at high risk of having

children with a genetic birth defect. even so, for some

people it raises questions of morality.

it is now possible to see a live, developing fetus with

the aid of an endoscope, a fiberoptic device. in fetoscopy,

sound waves are pulsed across the mother’s uterus. images

of parts of the fetus, umbilical cord, or placenta show up

on a computer screen that is connected to the endoscope

(Figure 17.22). a sample of fetal blood often is drawn at the

same time in order to diagnose blood cell disorders such as

sickle-cell anemia and hemophilia.

all three procedures pose some risk for the fetus,

including infections, punctures, or miscarriage. With

CvS there also is a slight chance the forthcoming child

will have missing or underdeveloped fingers or toes.

Figure 17.21 Animated! Amniocentesis is a common

prenatal diagnostic tool.

Fran Heyl Associates

image on the

ultrasound screen

Saturn Stills/Science Source

A Pulsed sound

waves guide

endoscope to

umbilical cord

D Blood sample

withdrawn by

syringe

B Placement of needle with

help of fiber optics

C Needle punctures tiny

fetal vein in the cord

Figure 17.22 Fetoscopy gives a direct view of a developing

fetus in the womb.

Lennart Nilsson from

A Child Is Born

, © 1966, 1977 Dell Publishing Co., Inc.

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