The blood-brain barrier is a semi-permeable lining that prevents most substances from passing into the brain, according to Washington University.

Your baby's development as an embryo begins with fertilization of the egg by a sperm, followed by rapid cell division. Initial division results in the formation of a ball of cells that then folds, divides, changes and grows into your baby with a genetically determined set of characteristics and appearance.

Nothing is cuter than a bouncing baby boy, but if that baby is bouncing in a jumper, his development may be hampered. Baby jumpers are designed to allow a baby to bounce by using his toes to push off from the ground -- a motion that delights most babies.

Cradle cap, also known as "seborrheic dermatitis" or "seborrhea," is basically the baby version of dandruff. Although cradle cap usually resolves itself within your baby’s first year, you may want to help alleviate the symptoms.

During your pregnancy, your baby will grow from a microscopic fertilized egg to an average of about 8 lb at birth. Fetal development follows predictable stages and milestones, with some individual variance.

If your 5-month-old baby has been to a handful of well-baby visits since his birth, the doctor has plotted your baby's weight length and head circumference.

Though babies can sleep for up to 16 hours in a day, these periods of sleep often last for only two to four hours at a time depending on the child's feeding schedule. An area of concern for most new parents is the timeframe in which their baby will develop the ability to sleep uninterrupted through the night.

Every baby gains weight at a different rate, but most follow a pattern falling within a set range for their age. These ranges are found on growth charts used by health care professionals who see infants and children for periodic well-child checkups.

During the first 12 months of life, a baby goes through periods of rapid growth. MedlinePlus explains that well-child checkups are most frequent during this first year to monitor a baby's growth to make sure she is on track with standard measurements, including weight.

Babies born at less than 37 weeks' gestation are preterm, or premature. According to the author of “When You're Expecting Twins, Triplets, or Quads,” Dr. Barbara Luke, roughly 10 percent of single baby and 50 percent of twin pregnancies are premature.

Every baby grows differently, and a specific weight is not as important as is her overall rate of growth, according to HealthyChildren, a website associated with the American Academy of Pediatricians. Your baby's weight gain depends on different factors such as her diet, genetics and overall health.

All parents wonder if their child has reached milestones at certain times in her development. Though children develop at differing rates, there are some things most 9-month-old babies typically can do. The American Academy of Pediatrics states that between 8 and 12 months a baby will become increasingly mobile.