Why Are Some Babies Born Bigger Than Others?

One of the first question new parents ask is how much their baby weighs. A perfectly normal full-term newborn can weigh from 5 pounds, 8 ounces, to 8 pounds, 13 ounces, the KidsHealth from Nemours website explains. Babies who weigh less or more than normal can develop a higher incidence of health problems at birth and possibly later in life.

Length of Pregnancy

Because babies gain about half a pound a week in the last month of pregnancy, according to MayoClinic.com, it stands to reason that the longer a baby stays in the womb, the heavier he'll be. This is true until the placenta starts to age, normally around two weeks after your due date. When this happens, your baby might actually start to lose weight. Many post-mature babies have loose, wrinkled, peeling skin and an emaciated look, because they've lost subcutaneous fat as the placenta became less effective at delivering nutrients to them.


A number of genetic and congenital issues can affect your baby's weight. A baby who has large, heavier parents has a higher chance of being larger and heavier at birth. Firstborns often weigh less than their siblings. A baby with a genetic disorder such as Down syndrome or a congenital heart condition might also weigh less at birth. Congenital infections such as rubella, syphilis or cytomegalovirus can also cause intrauterine growth restriction and smaller-than-normal babies 2.

Maternal Nutrition

Nutrition is possibly the most important determinant in fetal weight gain, after genetic factors, according to Dr. Léna Coïc, chief medical editor for the Medical Education Network 3. In most cases, the baby takes what he needs from mom to grow, even if mom suffers as a result. But women who suffer from very poor nutrition generally have smaller babies than women who eat well, while obese women who gain large amounts of weight during pregnancy have bigger babies. Both body mass index and weight gain during pregnancy influence fetal growth.(ref 3)

Maternal Disorders

Several disorders can affect your baby's growth before birth. Diabetes -- whether gestational, Type 1 or Type 2, can have a profound effect on newborn weight. In most cases, babies born to diabetic moms weigh more than most babies, because they have a higher level of glucose in their bloodstream.(ref 1) If you have a problem that affects the placenta, your baby will get fewer nutrients through it, which means he will probably weigh less than most babies of the same gestational age. High blood pressure and maternal heart disease are also risk factors for a low-birthweight baby.(ref 2)

Other Factors

If you drink alcohol heavily, smoke cigarettes or use recreational drugs, your baby could weigh less than other newborns 1. Moms who live at high altitudes also have smaller babies, according to MedlinePlus.(ref 2)