Cradle cap (the common name for seborrheic dermatitis) affects many babies and looks like dark red bumps accompanied by yellowish crusty flakes of skin on the head, neck, eyebrows and shoulders. It can also affect the nappy area.
It can last right through to when the baby is a year old or so and is caused when the glands responsible for depositing natural oils on the skin (the sebaceous glands) go a bit mad and overdo their job. The resulting excess oil causes old skin cells to get trapped on the head and build up.
Babies aren’t bothered by the condition – it doesn’t itch or cause pain, it just looks a bit unpleasant! It’s safe to leave it alone and wait it out, but if you want to try to treat it you can try rubbing your child’s head (gently) when washing his hair and perhaps massaging with a little olive oil followed by shampooing to give it all a good cleanse. To avoid cradle-cap developing in the first place, don’t be afraid to give your baby’s head a good (but gentle) rub when washing his hair to displace the built-up oil and skin cells.
If you are still concerned, your GP can prescribe special shampoo that contains selenium, or even tar. However, such shampoos are not terribly kind on the eyes and will sting if you’re not careful enough. It’s something that will disappear with time anyway and is nothing to be worried about.