Until yesterday, most people would never have heard of hyperemesis gravidarum (HG), the acute form of morning sickness so publicly affecting Catherine of Cambridge (don’t even get me started on the wording of the Guardian article about her pregnancy this morning). It’s a relatively rare complication of pregnancy (0.5-2% of women will suffer from it), and may result in severe nausea and vomiting for the first trimester, or even the whole pregnancy. Arguably, the most famous sufferer of this condition is NOT Kate Middleton, but Charlotte Brontë, who may have died of its complications - severe dehydration and liver disease, as well as rapid weight loss that poses a risk to the mother and the child.
It’s hard to say why some women get this extreme form of morning sickness - it is likely that a large epidemiological cohort study is needed to get to the bottom of this condition. This is where genetics could prove helpful to women like the Duchess and Charlotte Brontë. If a woman has HG in pregnancy, her identical twin is twice as likely to develop HG in pregnancy as a non-identical female twin would be. HG clusters in families - if your sister has HG, you are more 17 times more likely than average to also have HG if you become pregnant, and the risk also increases if your mother had HG. HG is also found more often in women with certain genetic conditions - all adding up to the fact that Catherine and Charlotte may not merely be unlucky in the occurrence of this condition; they may have been genetically at risk their whole lives. The current media attention on hyperemesis gravidarum may be what’s needed to spur a genome-wide association study on this condition. Given the low prevalence of HG, it would probably need to be a world-wide collaboration from several cohorts. If we knew what genetic variants predisposed women to this condition, you could be screened for it when you go for your first scan - or if a female relative has suffered from it - and the correct treatment could be given more quickly.
I may have to become “Disgruntled Scientist of Cambridge” and send this excellent research paper (http://www.ncbi.nlm.nih.gov/pubmed/20974461) to any ignorant hack who publishes an article suggesting this condition is in some way Catherine’s fault.