Around now your baby will start to develop vernix caseosa; a greasy, creamy, somewhat cheesy substance that coats the entire body and protects the skin from the amniotic fluid. Up until now, your baby has been fine floating in the fluid, but think about what happens when you have a bath. After an hour or two you start to look a bit like a prune, right? Well, think of what your baby would look like after a 40 week bath! The coating stops the skin from drying out and keeps it moisturized, but what many Moms don't know is that it actually does a lot more.
The vernix caseosa coating contains a protein that acts as a sort of topical antibiotic. This helps to protect your baby from disease both in and out of the womb as the natural antibiotic is absorbed by the skin, providing protection for your newborn in those first few weeks when they are most vulnerable to infection.
If your baby is born on time, there may be a little of this coating left on certain areas of the skin, although there shouldn't be too much and it's easy to clean off. If your baby comes early, they may be covered in much more of the substance, and if your baby arrives past your due date it should all have completely disappeared.
Your baby is practicing all manner of facial expressions now, especially opening and closing the mouth. If your baby is awake during your ultrasound you may see this happening, which can make them look a bit fish-like! You'll endure all sorts of bizarre expressions from your newborn, but should be rewarded with ear-to-ear grins from about 4 or 5 weeks of age!