Infant acne affects around 40% of newborns according to the NHS website, although in my experiences with the hundreds of babies I have worked with, I would say the figure is much higher than that and very common with the majority of babies. I have only had 2 babies in my whole career (including my own 3 children,) who weren't affected by it.
It usually begins around two to three weeks after birth and in some cases can last until your baby is two to four months old, although in most instances it is at its worst for a week or two and then gradually improves with each day.
The definite cause is unknown, although it is believed that it is the mothers hormones still circulating in the babies system that stimulate their sweat glands and cause pimples to develop.
Another factor that contributes is that dirt and germs in the air, get into the baby's still developing pored and cause the angry rash and white headed spots.
The acne usually only affects the face and neck, although it can sometimes spread to the top of the torso. This is because the face and the neck are the most sensitive parts of your baby's body, so will always be affected first, particularly as they are usually uncovered and subject to the dirt and germs in the air.
DO NOT be tempted to squeeze the spots or pick them, as this can lead to infection. Wash your baby's face with cool boiled water and clean cotton pads. No soap should be used.
If the acne is really bad and dries out your baby's skin, then get your GP to take a look;they may prescribe a cream if the spots/rash doesn't seem to be improving.
A rash on the face and neck that doesn't improve can be a sign of an allergy.
All 3 of my children suffered from baby acne. My middle child had it particularly badly and got to the stage where his little face and head were red raw. As a parent it's horrible to see your child suffering like that, even while you know it's just a stage they commonly go through. Other people's comments did not help and there were lots of times I had to bite my tongue. I ended up taking my baby to the doctor and after trying various creams and then finally a mild steroid cream, the rash gradually got better and disappeared.