The Duchess of Cambridge's doctors are already well-practised at treating her hyperemesis gravidarum.
With both her previous pregnancies, Kate suffered from the rare condition which causes severe vomiting.
It can lead to dehydration, weight loss and a build-up of toxins in the blood or urine called ketosis.
Experts warn the condition often gets worse with subsequent pregnancies, but those who have experienced it before can be put on medication as soon as they get a positive pregnancy test.
When expecting Prince George in 2012, Kate spent three nights in King Edward VII's Hospital in central London before being discharged.
With Princess Charlotte, the Duchess was cared for at home, as is the case with her third pregnancy.
Nicola James, trustee of the charity Pregnancy Sickness Support, said: "It's highly likely that if you have had it before you will have it again and it's generally worse in subsequent pregnancies. It starts earlier and is usually more severe.
"But if it was treated well last time and you can get things in place, you might be all right.
"Some women even go as far as seeing their consultant and being put on medication as soon as they get a positive pregnancy test."
Hyperemesis gravidarum (HG) affects 3.5 in every 1,000 pregnant women and can cause them to vomit blood.
It can be treated with anti-sickness tablets and by giving fluids intravenously.
Normal morning sickness (known medically as nausea and vomiting in pregnancy, or NVP) affects around 70% of pregnant woman, and causes some degree of vomiting and discomfort commonly during the first trimester (12 weeks) of pregnancy.
But those suffering from HG can be constantly sick morning, noon and night and unable to keep any food or drink down.
Other symptoms include dehydration, low blood pressure, tiredness, dizziness and weight loss, and HG can go on for much longer too, sometimes, although rarely, for the entire pregnancy.