It affects about eight out of 10 women, starting around week six, and for most it stops around12-14 weeks - but one in 10 unlucky ones still feel sick after 20 weeks. You're more likely to get morning sickness if you're having a multiple pregnancy.
It's not known what causes morning sickness, but it's thought that those pesky hormonal changes triggered by pregnancy are probably partly responsible.
Some women are sick, others feel nauseous without actually being sick. At a point when you may already feel tired and emotional and have a heightened sense of smell, it can be very trying - especially if most people don't even know you're pregnant yet.
It's hard to describe how morning sickness feels, but this Mumsnetter has a go: "A weird feeling of having two stomachs - one that wants to throw up and burp constantly, and the other that is constantly ravenous and thinking about food all the time."
Gross and debilitating as it is, morning sickness won't usually harm you or your baby and, in fact, if you do feel sick, research suggests you're less likely to have a miscarriage. (Every cloud and all that.)
If you can't keep anything down for days on end and you're becoming dehydrated, then you may have hyperemesis and you'll need expert help from your doctor or midwife, who'll want to monitor you a bit more closely.