The black, tar-like stuff called meconium is made up of mucus, fluid from the womb, and anything else they digested while inside mom. But it doesn't yet have the gut bacteria that make poop smelly. As soon as you start feeding a baby, bacteria will start colonizing their intestines. After a day or so, bowel movements become green, yellow, or brown -- with that familiar odor.
Sometimes infants stop breathing.
Likely when they're sleeping, they may pause without a breath for 5 to 10 seconds -- just enough time to make a new mom or dad panic. Irregular breathing is normal. (But if your baby stops breathing for a longer time or turns blue, it’s a medical emergency.) When babies are excited or after crying, they may take more than 60 breaths in a minute.
Their tonsils have taste buds.
Although a baby has about the same number of taste sensors as kids and younger adults, they cover more areas, including the tonsils and the back of the throat. A newborn can taste sweet, bitter, and sour, but not salty (until around 5 months). It's a matter of survival: Breast milk is sweet, while bitter and sour may be harmful. When they start on solid food, they'll tend to like the same things mom ate while pregnant and breastfeeding.
They cry without tears, at first.
Babies start crying around 2-3 weeks, but tears don't show up until they're about a month old. Late afternoon and early evening are prime fussing time. Often, it's for no reason, and nothing you do will help.
"Peak crying" is around 46 weeks after gestation, or age 6-8 weeks for full-term babies. After 3 months, the storm has usually passed. (Preemies tend to be older, since they were born early.)
Newborns have breasts.
When they’re first born, both boys and girls can look like they have small breasts. These may even leak milk! Don't squeeze the firm little lumps though. They form because babies absorb estrogen from mom, and they'll usually go away within a few weeks. Baby girls could also have a mini period or vaginal discharge that lasts a few days.
They like to face right.
Only 15% of newborns prefer to turn their head left when lying on their back. It seems to be related to a gene, like having dimples. This bias lasts for a few months, and it may help explain why more people are right-handed, too.
They have more of certain brain cells.
Although a baby's brain will get bigger -- more than doubling in size the first year -- it already has most of the nerve cells that carry electrical messages. Many of these neurons won't get replaced as they die, so adults have fewer of them. The connections between cells get "trimmed" as babies get older, which helps them focus but also cuts back on creativity.
Baby boys get erections.
It often happens just before they pee. (Consider it your warning to take cover during a diaper change!) We don't know exactly why, but it's nothing to be worried or ashamed about. You might even see one on an ultrasound, before they are born.
His penis may look large at birth, and that's normal, too. Their hormones and mom's play a role, as well as bruising and swelling from the birth process.
They can scare themselves.
It doesn’t take much to startle a newborn: a loud noise, strong scent, bright light, sudden motion, even their own cries. You'll know it's happened when they fling their arms out to the sides, hands open, then quickly closes up and tucks back in toward their body. This Moro reflex might have developed as a warning signal that a young monkey was off-balance, so mom could prevent a fall.
Some birthmarks will disappear.
"Stork bites" or "angel kisses" (a pink or red area often on the forehead, eyelids, bridge of the nose, or back of the neck) and Mongolian spots (flat, bluish patches that look like ink stains on the back or bottom) usually fade within a few years. We don't know what causes them.
A bright-red raised "strawberry" hemangioma comes from fast-growing blood vessels. These birthmarks appear over several weeks and can take years to go away.