In this condition, the pylorus, the passage that leads from the stomach into the small intestine, is narrow because of a thickening of the muscle. The cause is unknown, and it is more common in boys than in girls.

Symptoms usually first appear at two to four weeks, though they can appear earlier or later. The stomach contracts powerfully in an attempt to force a buildup of food through the narrow pylorus. This is impossible, however, and the contents of the stomach are vomited up so violently that they may be propelled up to 3 feet (1 meter) – projectile vomiting. The baby may also suffer constipation and dehydration. Ultrasound and a barium swallow may be used to confirm the diagnosis. A simple operation to widen the pylorus can be performed, curing the condition completely.

This is by far the most common of a range of conditions called trisomies, in which one pair of chromosomes has an extra chromosome, making three. In Down syndrome, there are three number 21 chromosomes. Affected infants characteristically have small features, a tongue that protrudes, and slanting eyes with folds of skin at their inner corners. They tend to be rather floppy infants and have short, wide hands. They may also suffer from congenital heart disease.

Down syndrome babies usually mentally handicapped, though the degree of handicap varies widely; many are near normal. They are usually affectionate and happy children. With careful attention and early education, they often do very well, and some manage to live independently as adults.

In a very small number of male babies – about 0.3 percent – there is an abnormality in the position of the urethral opening on the penis. In epispadias, the opening is on the upper surface of the penis, and the penis may curve upward; in hypospadies, the opening is on the underside of the glans (head), and the penis may curve downward. In rare cases, the urethral opening lies between the genitals and the anus, and the genitals may appear to be female. Surgery can be carried out in the preschool years to correct the defect, allowing normal passage of urine and, in later life, normal sexual intercourse. Neither of these conditions, even in the severe form, causes infertility.

A doctor will check your baby’s hips for excessive mobility as part of routine tests after birth. Treatment such as manipulation and splinting can prevent trouble in later infancy. In severe cases, an operation may be needed.

A baby with respiratory distress syndrome (RDS) may stop breathing for a short time. This is called apnea. Although it sounds very frightening, apnea is not uncommon, and most babies start breathing again after gentle stimulation such as a top or a stroke. Other respiratory problems can arise from fluid inhaled into the lungs or a lack of surfactant – a substance produced in the lungs that keeps them from collapsing. If a baby’s do not have enough surfactant coating, they don’t expand as well as they should. This can cause the smaller air sacs to collapse, leading to hyaline membrane disease, a condition common in babies born before 31 weeks.

Babies suffering from any of these complications can be given oxygen either by way of a face mask or by a small tube inserted directly into the windpipe and attached to a ventilator.

About 7 percent of all babies born in the U.S. weigh less than 5½ pounds (2.5 kilograms), either because they are premature or for other reasons. All low birth-weight babies need special care.

When we say a baby is premature we mean that he has not yet matured to the point where he can cope easily outside the security of his mother’s uterus. Although the chances today that a premature or low birth-weight baby will survive and thrive are vastly improved in comparison to our mother’s generation, it’s still a difficult experience to see your baby being taken away to a special care or intensive-care unit immediately after the delivery. Understanding why a baby needs special treatment for a few days or weeks will help lessen your anxiety. Premature babies have very weak muscle tone and don’t move much. They often have calcium and iron deficiencies, as well as low blood-sugar levels. If they are very premature their eyes may still be sealed. They have very red and wrinkled skin. Their heads are disproportionately large in comparison to the rest of the body, and the bones in their skull are soft. They are more than usually prone to jaundice.