I have just found this on the New England Journal of Medicine in May 2013:
The odds of a medical emergency are 1 per 604 flights, or 16 per 1 million passengers.
Planes had to be diverted for emergency help in only 7 percent of cases.
Doctors were on board and volunteered to help in 48 percent of cases; nurses and other health workers were available in another 28 percent. Only one-third of cases had to be handled by flight attendants alone.
The most common problems: Dizziness or passing out (37 percent of cases); trouble breathing (12 percent) and nausea or vomiting (10 percent).
About one-fourth of passengers were evaluated at a hospital after landing and 9 percent were admitted, usually with stroke, respiratory or cardiac symptoms.
Out of nearly 12,000 cases, a defibrillator was applied 137 times, including in 24 cases of cardiac arrest, where the heart had stopped. (Sometimes defibrillators are used to analyze an irregular heart rhythm to help doctors figure out what to do, not necessarily to deliver a shock.)
Of the cases in this study, only 36 deaths occurred, 30 of them during the flight and the others after landing.
Pregnancy-related problems were generally rare — 61 cases, in this study — and two-thirds of them involved women less than 24 weeks along with possible miscarriages. Air travel is considered safe up to the 36th week, or the last month, of pregnancy. Only three cases of women in labor beyond 24 weeks of pregnancy led to a plane being diverted.