This is called intrauterine growth restriction (IUGR). It may happen because of problems with the placenta, the mother’s health, or birth defects. Most very low birth weight babies who have IUGR are also born early. They are usually very small and physically immature.

What causes low birthweight? The primary cause is premature birth, being born before 37 weeks gestation; a baby born early has less time in the mother’s uterus to grow and gain weight, and much of a fetus’s weight is gained during the latter part of the mother’s pregnancy.

What causes low birth weight? Low birth weight is most often caused by being born too early (premature birth). That means before 37 weeks of pregnancy. A premature baby has less time in the mother’s womb (uterus) to grow and gain weight.

With the exception of a small minority of low birth weight children with mental retardation and/or cerebral palsy, the developmental sequelae for most low birth weight infants include mild problems in cognition, attention, and neuromotor functioning.

The average birth weight for babies is around 7.5 lb (3.5 kg), although between 5.5 lb (2.5 kg) and 10 lb (4.5 kg) is considered normal. In general: Boys are usually a little heavier than girls.

Babies with a very low birth weight have a greater risk of developing problems. Their tiny bodies are not as strong as babies of normal weight. They may have a harder time eating, gaining weight, and fighting infection. They have very little body fat.

The most common reason why a baby is smaller than average — weighing less than 2.5kg at birth — is prematurity (being born before 37 weeks’ gestation). The earlier the baby is born, the smaller they are likely to be. This is because the baby will have had less time in the womb to grow.

Consider trying these diet changes to gain weight more slowly:

1. Eat the appropriate portion size and avoid second helpings.
2. Choose low-fat dairy products.
3. Exercise; consider walking or swimming on most if not all days.
4. Use low-fat cooking methods.
5. Limit sweets and high-calorie snacks.
6. Limit sweet and sugary drinks.

For those of you who have a thing for math, here’s the equation: Birth weight (g) = gestational age (days) x (9.38 + 0.264 x fetal sex + 0.000233 x maternal height [cm] x maternal weight at 26.0 weeks [kg] + 4.62 x 3rd-trimester maternal weight gain rate [kg/d]] x [number of previous births + 1]).

A baby that weighs more than 10 pounds is more likely to be delivered via C-section, Simhan said. But even then, there are risks of bleeding and delivery complications.

The medical term for big baby is macrosomia, which literally means “big body.” Some researchers consider a baby to be big when it weighs 4,000 grams (8 lbs., 13 oz.) or more at birth, and others say a baby is big if it weighs 4,500 grams (9 lbs., 15 oz.) or more (Rouse et al. 1996).