Prenatal development encompasses the 40 weeks of pregnancy. After conception, a zygote forms that rapidly changes into a blastocyte, embryo and finally a fetus. Every week of a woman’s pregnancy, the developing infant undergoes great changes as the body forms and takes shape. During the first three months (trimester) of pregnancy, the most dramatic changes occur. A ball of cells divides into three different types of tissue, which eventually divide to form the different body parts and organ systems. By the end of the first trimester, the fetus has the general appearance of a baby, with head, arms, legs and functioning organ systems.

Many problems that eventually become birth defects begin with malformations in the first trimester. For example, neural tube defects occur when the spinal cord fails to close properly. It may cause spina bifida or other more severe birth defects. Some abnormalities that occur are so severe that the pregnancy cannot be sustained and a miscarriage occurs. Other birth defects are the results of genetic defects, such as Down syndrome or muscular dystrophy. The mother’s actions during pregnancy may also affect the fetus, especially if she smokes, drinks alcohol or uses recreational drugs. Some birth defects can be identified and repaired in infancy.

During the second and third trimesters of pregnancy, most development is related to growth, weight gain and the outer appearance of the fetus. Some problems during these later parts of pregnancy may result in premature birth. Babies born before full term may be healthy, but may also be at increased risk for problems such as respiratory trouble, cerebral palsy and developmental delays. Even premature infants born with no apparent problems will develop more slowly than their full-term counterparts. When gauging an infant’s milestones, parents should measure from the baby’s actual due date. For example, a child born one month prematurely should not be expected to meet six-month-old milestones until he or she is at least seven months old.

The milestones of development during pregnancy are well defined. A woman’s obstetrician can monitor her progress throughout her pregnancy and can usually tell when certain milestones of weight or growth are not being met. Various prenatal tests may identify and rule out certain genetic problems.

Infancy development covers the period from birth to 12 months of age. During this stage, a baby will experience rapid changes, including physical growth and brain development. For example, the baby begins to breathe at birth, developing the respiratory system. The cardiac system has functioned during prenatal development, but changes occur in the heart after birth so that blood is oxygenated and transported throughout the body. Other than regular growth, physical changes include the fusing of the bones in the baby’s skull and the eruption of teeth in the first year.

An infant’s brain also grows rapidly, as does the neural connections inside the brain. In the first year, infants develop greater acuity in their senses and begin to understand and use methods of communication like crying. From early on they recognize speech, especially their parents, and eventually can understand the meanings of some words, long before they can speak coherently. Babies can speak a few words by their first birthday. They also begin to recognize that things continue to exist even if they are out of sight.

Infants also learn about social connections in their first year. They respond to their parents and other caregivers and learn to prefer their presence. They learn to smile, play and interact with others. By about the age of eight months, infants may develop separation anxiety when they are separated from their parents or primary caregivers.