ed that about
one-third of the pregnant women were free from sickness
throughout pregnancy, 45 per cent. were free during the first
three months. When sickness occurred it began in 70 per cent. of
cases in the first month, and was most frequent during the second
month. The duration varied from a few days to all through.
Between the ages of 20 and 25 sickness was least frequent, and
there was less sickness in the third than in any other pregnancy.
(This corresponds with the conclusion of Matthews Duncan that 25
is the most favorable age for pregnancy.) To some extent in
agreement with Gueniot, Giles believes that the vomiting of
pregnancy is "one form of manifestation of the high nervous
irritability of pregnancy." This high nervous tension may
overflow into other channels, into the vascular and excretory
system, causing eclampsia; into the muscular system, causing
chorea, or, expending itself in the brain, give rise to hysteria
when mild or insanity when severe. But the vagi form a very ready
channel for such overflow, and hence the frequency of sickness in
pregnancy. There are thus three main factors in the causation of
this phenomenon: (1) An increased nervous irritability; (2) a
local source of irritation; (3) a ready efferent channel for
nervous energy. (Arthur Giles, "Observations on the Etiology of
the Sickness of Pregnancy," _Transactions Obstetrical Society of
London_, vol. xxv, 1894.)

Martin, who regards the phenomenon as normal, points out that
when nausea and vomiting are absent or suddenly cease there is
often reason to suspect something wrong, especially the death of
the embryo. He also remarks that women who suffer from large
varicose veins are seldom troubled by the nausea of pregnancy.
(J.M.H. Martin, "The Vomiting of Pregnancy," _British Medical
Journal_, December 10, 1904.) These observations may be connected
with those of Evans (_American Gynaecological and Obstetrical
Journal_, January, 1900), who attributes primary importance to
the undoubtedly active factor of the irritation set up by the
uterus, more especially the rhythmic uterine contractions;
stimulation of the breasts produces active uterine contractions,
and Evans found that examination of the breasts sufficed to bring
on a severe attack of vomiting, while on another occasion this
was produ