Some women have it throughout their entire pregnancy, which is horrific and it is such a debilitating condition that they then have to stay of work as they have no energy. The constant loss of fluids via vomiting results in the women losing a lot of weight and the fetus therefore does not receive enough nutrition, sometimes resulting in small for gestational age babies (those which are not born above 2.5kg at birth if they are full term, which is 37 weeks gestation).
It often resolves itself after 12 weeks, and it is a sign of an increase in a certain hormone (HCG) which is the hormone that sustains a pregnancy. Increased levels of the hormone are associated with multiple pregnancies, but I am not saying Catherine is expecting twins. It is a good sign if a woman is quite sick during the first few weeks, as it means the hormone levels are increasing thus sustaining a pregnancy. It is just awful to experience nausea and vomiting all day.
I am a midwife, and true hyperemesis is rarely seen and if it does occur it is treated with IV fluids and antiemetics (anti sickness medication) either orally, via injections or intravenously. It is often gone by around 12-14 weeks, and women start to feel much better by then.
