d="p0005">Morning sickness is common in early pregnancy but is of unknown aetiology.
dd><dt class="label">•dt><dd>d="p0010">Its severe form, Hyperemesis Gravidarum (HG), requires hospital admission.
dd><dt class="label">•dt><dd>d="p0015">Using Swedish register data, we show that HG is more common among low education mothers.
dd><dt class="label">•dt><dd>d="p0020">We confirm a strong girl bias among live births following HG in pregnancy.
dd><dt class="label">•dt><dd>d="p0025">HG may be a proximate mechanism for the so called Trivers–Willard hypothesis.
dd>dl>