Many things affect the baby's size at birth. Some do not imply a problem and smaller weights are not always indicative of growth retardation. Girls,for example ordinarily weigh a few ounces less than boys. A woman's first child usually weighs less than her subsequent babies. Mothers who them selves were large newborns tend to have large babies. The size of the baby's father also plays a role in its birth weight.

Heredity

Heredity plays a large role in fetal size. Ordinarily, parents descended from lineage of big people breed infants of large size, and those with small parents and grandparents produce babies of less than average birth weight, The size of members of the paternal line is at least as important as of those the maternal line.

Gestational Age

Of course, birth weight depends a lot on gestational age at birth how many pregnant the mother is when the baby is born. A baby .who weights four pounds because she or he was born prematurely at 32 weeks is at an appropriate weight for its gestational age. A baby who is born at 32 weeks and weighs two pounds is small for that gestational age as well as premature. This baby will have the most problems.

Maternal Weight and Weight Gain

Nutrition is important. How much the mother weighed when she began pregnancy and how much weight she gained during pregnancy can influence birth weight. The exact relationship of maternal weight gain to infant weight is not entirely clear or simple. Among malnourished women, low weight can lead to poor fetal growth and low birth weight. This is more common in developing nations than in industrialized countries, but certain populations in industrialized countries are at risk for poor weight gain and poor infant growth. These include young adolescents and women with eating disorders. Excess maternal weight gain may be a risk for the baby becoming an extra-large baby, which can result in problems such as difficult birth. Maternal obesity and excess weight gain may also result in maternal complications.

Race

Birth weight also varies according to race. African-American infants tend to be smaller than white infants of the same gestational age. Experts therefore advise that African-American women should have weight gains in pregnancy reflecting the upper limit of the recommended range of weight gain (twenty-five to thirty-five pounds). Asian women have smaller babies than white women, but this may simply reflect smaller size among Asian people.

Mother's Age

Although research shows conflicting results, young adolescent women also tend to deliver smaller babies and should gain toward the higher end of the recommended spectrum.