Essentials of Anatomy and Physiology

485

BOX21–5 PREMATURE BIRTH

been successfully treated and eventually sent home

with their parents. Whether there will be any long-

term detrimental effects of their very premature

births will only be known in time.

Medical science may have reached its maximum

capability in this area. In a fetus of 20 weeks, the

cells of the alveoli and the cells of the pulmonary

capillaries are just not close enough to each other

to permit gas exchange. At present there is no

medical intervention that can overcome this

anatomic fact.

The lowest birth weight with subsequent sur-

vival is believed to be 244 grams (8.6 oz) in

September 2004. The little girl had a gestational

age of 26 weeks.

A premature birth is the spontaneous delivery of an
infant before the end of the gestation period, or the
intentional delivery (for reasons concerning the
health of the mother or fetus) before that time. A
premature infant may also be defined as one who
weighs less than 2500 grams (5.5 pounds), though
gestation time is more important than birth weight.
In terms of gestation time, an 8-month infant
has a very good chance of surviving if good medical
care is available. Infants of 7 months gestation or
less often have respiratory distress, because their
lungs have not yet produced enough surfactant to
permit inflation of the alveoli. This is hyaline mem-
brane disease, which was described in Chapter 15.
Infants as young as 23 weeks of gestation have

Amniotic sac

Amniotic fluid

Placenta

Umbilical cord

Urinary bladder

Symphysis pubis

Vagina

Mucus plug

Cervix

Anus

Rectum

Spine

Figure 21–6. Full-term fetus positioned head down within the uterus.

QUESTION:Why is it beneficial that the fetus is positioned head down?