18 weeks

Six inches long this week and about eight ounces in weight, your baby is the size of a large mango and is now covered in a protective substance called vernix. Arms and legs are finally in proportion, neurons are now connected between the brain and muscles, and cartilage throughout the body is turning to bone.
Six inches long this week and about eight ounces in weight, your baby is the size of a large mango. Your little action figure is able to choreograph Matrix-like moves. Arms and legs are finally in proportion, neurons are now connected between the brain and muscles, and cartilage throughout the body is turning to bone. All these upgrades combine to give your baby more control over limb movements. Which explains all that kicking, stretching, and bodysurfing (or rather bellysurfing) you've possibly started feeling by now.
Something else going on this week: Your baby is getting a cheesy varnish. Say what? Yup — a protective substance called vernix caseosa (vernix is the Latin word for varnish; caseosa is cheese) now covers your baby's skin. It's greasy and white and is made up of lanugo (that downy hair), oil from your baby's glands, and dead skin cells. This waxy "cheese" may not sound too appetizing or attractive, but it's there for good reason: Vernix protects your baby's sensitive skin from the surrounding amniotic fluid. Without it, your baby would look very wrinkled at birth (sort of what you'd look like if you soaked in a bath for nine months). Some babies — especially those born early — will still be covered with vernix at the delivery, so you might get a look at your baby's first anti-wrinkle cream.