biorhythmsPhysiological functions that
vary in a rhythmic way. Most biorhythms
are based on a daily, or circadian (24-
hour), cycle. Our bodies are governed
by an internal clock, which is itself regu-
lated by hormones. Periods of sleepiness
and wakefulness may be affected by the
level of melatoninsecreted by the pineal
gland in the brain. Melatonin release is
stimulated by darkness and suppressed
by light. Cortisol, secreted by the ad-
renal glands, also reflects the sleeping
and waking states, being low in the
evening and high in the morning.
biotechnologyThe use of living organ-
isms such as bacteriain industry and
science (for example, in drug production).
biotinA vitamin of the B complex (see
vitamin B complex) that is essential for
the breakdown of fats.
biphosphonate drugsSee bisphos-
phonate drugs.
bipolar disorderAn illness, commonly
known as manic–depressive illness,char-
acterized by swings in mood between
the opposite extremes of severe depres-
sion and overexcitability.
birthSee childbirth.
birth canalThe passage through the pel-
vis from the cervix (neck of the womb)
to the vaginal opening through which
the baby passes during childbirth.
birth controlLimitation of the number
of children born, either to an individual
or within a population. Family planning
allows men and women to choose if
and when to have children; contracep-
tioncan prevent unwanted pregnancies.
birth defectsAbnormalities that are
obvious at birth or detectable early in
infancy. Also called congenital defects,
they encompass both minor abnormali-
ties, such as birthmarks, and serious
disorders such as spina bifida.
Causes include chromosomal abnormal-
ities, genetic defects, drugs taken during
pregnancy, exposure to radiation, and
infections. In some cases, the cause of a
defect is unknown. Defects that are due
to chromosomal abnormalities include
Down’s syndrome. Some defects, such
as achondroplasiaand albinism,are usu-
ally inherited from 1 or both parents
(see gene; genetic disorders). Certain
drugs and chemicals (called teratogens)
can damage the fetus if the mother
takes or is exposed to them during early
pregnancy. Teratogenic drugs include
thalidomide (now rarely prescribed) and
isotretinoin, which is used in the treat-
ment of severe acne. Alcohol can affect
the development of the brain and face
(see fetal alcohol syndrome).
Irradiation of the embryo in early preg-
nancy can cause abnormalities. Very
small doses of radiation increase the
child’s risk of developing leukaemialater
in life (see radiation hazards).
Certain illnesses, such as rubella(Ger-
man measles) and toxoplasmosis, can
cause birth defects if they are contrac-
ted during pregnancy.
Brain and spinal cord abnormalities,
such as spina bifida and hydrocephalus,
and congenital heart disorders (see
heart disease,congenital) result from
interference with the development of
particular groups of cells. Other com-
mon defects include cleft lip and palate.
Ultrasound scanningand blood tests
during pregnancy can identify women at
high risk of having a baby with a birth
defect. Further tests such as chorionic
villus sampling, amniocentesis, or fetos-
copymay then be carried out.
birth injuryDamage sustained during
birth. Minor injuries, such as bruising
and swelling of the scalp during a vagi-
nal delivery (see cephalhaematoma) are
common. More serious injury can occur,
particularly if the baby is excessively
large and has difficulty in passing
through the birth canal. A breech deliv-
erymay result in injury to nerves in the
shoulder, causing temporary paralysis
in the arm. The face may be paralysed
temporarily if the facial nerve is trauma-
tized by forceps. Fractured bones are
another hazard of difficult deliveries,
but the bones usually heal easily. (See
also birth defects; brain damage.)
birthmarkAn area of discoloured skin
present from birth, or very soon after-
wards, such as moles, freckles, and other
types of melanocytic naevus(various flat,
brown to blue-grey skin patches), straw-
berry marks, and port-wine stains. The
last 2 are types of haemangioma(mal-
formation of blood vessels). Strawberry
marks often increase in size in the firstBIORHYTHMS BIRTHMARK
B