Webster's New World Medical Dictionary

natural family planning
Birth control without
the use of contraceptive medications or barrier
methods. There are several natural family planning
methods. The most common natural family plan-
ning method is the use of basal temperature to
detect ovulation, accompanied by abstention from
intercourse during and after ovulation. Because a
sperm may live in the female’s reproductive tract for
up to 7 days and the egg remains fertile for about 24
hours, a woman can get pregnant within a substan-
tial window of time. Also known as the basal tem-
perature method, fertility awareness, periodic
abstinence, and the rhythm method. See also 
birth
control.
natural killer cell
A cell that can react against
and destroy another cell without prior sensitization
to it. Abbreviated NK cell. NK cells are part of our
first line of defense against cancer cells and virus-
infected cells. NK cells are small lymphocytes that
originate in the bone marrow and develop without
the influence of the thymus. An NK cell attaches to a
target cell, releases chemicals that breach its cell
wall, and causes it to lyse (break up).
natriuresis
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natural menopause
See 
menopause.
naturopath
A person who practices naturopathy.
A naturopathic doctor (ND) has been trained to
care for well and ailing patients by using naturo-
pathic methods. In some US states, NDs must com-
plete a program equivalent to the training received
by MDs, and they are therefore licensed to practice
medicine. In other states, the term ND is neither
defined nor regulated. See also 
naturopathy.