For many women, the elation of discovering they are pregnant can be slightly tempered by the first wave of nausea that soon follows.
Few expectant mothers escape morning sickness. According to NHS figures, around half of women suffer from it to some degree.
Doctors do not fully understand why some mothers-to-be get it and others don’t.
“We think that it is caused by the effects of the hormone Human Chorionic Gonadotropin (which is made by the developing placenta after conception) on an area of the brain (the chemoreceptor trigger zone) that plays a part in controlling the vomiting reflex,” says Mr Karl Murphy, a Consultant Obstetrician at The Portland Hospital.
“We believe that HCG makes this area more sensitive. It could be an evolutionary mechanism to ensure pregnant women don’t ingest poisons that could be harmful to the developing baby.
Morning sickness tends to start between six and eight weeks of pregnancy. But the idea that it just strikes in the morning is incorrect, says Dr Donald Gibb, Consultant Obstetrician at The Birth Company.
“It can occur all day but may be worse in the morning because your stomach is empty which heightens the feelings of nausea,” he says.
“The other misnomer is that it causes sickness – the majority of women I see have feelings of nausea and generally not feeling very good but aren’t actually sick.”
However, some women – the Duchess of Cambridge among them – develop a severe form of the condition called Hyperemesis Gravidarum (HG).
This is a relatively rare complaint, affecting around one in 200 pregnant women, and causes sickness repeatedly throughout the day.
“We think that in these women, their brains became extremely sensitive to the effects of HCG,” says Mr Murphy.
With HG, women can become so sick and nauseous that they are at risk of dehydrating and ultimately organ damage.
“This is usually diagnosed when a woman has ketones in her urine (an acid by-product of the body burning its own fat),” says Dr Gibb.
“If she is vomiting so much that body fat is being broken down, then this is a serious sign.”
In these circumstances, women require anti-sickness medication and, in some cases, hospitalisation to replace lost fluids and vitamins.
However, traditional morning sickness is rarely a cause for concern – and is actually a positive sign that pregnancy is progressing as normal.
“Some women do find that because they feel nauseous they eat less and lose weight but this is generally fine,” says Dr Gibb.
“The baby will take what it needs and even very slim women have some fat reserves.”
There are a number of self-help measures to alleviate the symptoms.
“If it is worse in the morning then eat something bland like a dry biscuit and then wait half an hour until you get up,” advises Dr Gibb.
There is also some - albeit mixed - evidence for taking ginger to help with nausea, for example, in the form of a tea, or by nibbling on crystallised ginger.
Acupuncture - either in the traditional sense or with acupressure bands worn around the wrist – may also help.
Women who suffer badly can ask their doctor for medication such as the anti-histamine promethazine which acts on the central vomiting reflex in the brain.
“It won’t help so much with nausea...but it can help stop the vomiting,” says Mr Murphy.
The good news is that by 12 weeks, as the amount of HCG stabilises, for most women morning sickness becomes a thing of the past.
By Lucy Elkins