There Are A Number Of Factors, Actually
What affects the baby's size at birth? Also, if my husband was a big baby (over 10 pounds), is our baby likely to be big? -- Beth in Portland, Oregon
The expert answers
The normal birth weight for babies born at term (between 37 and 42 weeks gestation) ranges from 2500 grams (5 pounds, 8 ounces) to 4000 grams (8 pounds, 13 ounces).
With such a wide range of what is considered normal, it is difficult to predict the birth weight of baby. There are several factors that contribute to this difficulty:
The parents' birth weights do factor into the equation, however the mother's birth weight seems to be more important.
Women with low pre-pregnancy weights (Body Mass Index less than 19) are more at risk for low birth weight (LBW) infants. In contrast, women who are overweight (Body Mass Index greater than 24) have an increased risk for larger birth weight babies.
For reasons that most likely are related to confounding medical problems, especially gestational diabetes, women of advanced maternal age (age 35 or older at the time of delivery) tend to have larger babies compared to younger women. Teen mothers are more at risk for low birth weight (less than 2500 grams) and very low birth weight (less than 1500 grams) deliveries.
Pre-existing medical problems
Chronic medical conditions (anemia, renal disease, pulmonary disease, diabetes mellitus, collagen-vascular disease, cardiac disease) typically result in low birth weight babies, most likely due to a reduction in the delivery of nutrients to the fetus.
Gestational Diabetes Mellitus (GDM)
The development of glucose tolerance during pregnancy, also known as gestational diabetes mellitus, allows for an increase in the availability of glucose to the fetus. The increase in the amount of "fuel" may lead to a larger baby.
Gestational age at birth
The majority of weight gain occurs during the third trimester, especially during the last four weeks prior to delivery. During the last month, weight gains of as much as one half pound per week are possible. Therefore, the time of delivery definitely has an impact on birth weight.
In pregnancies with multiple gestations (twins, triplets, etc.), the average birth weight is less than that of a singleton delivery.
Placental location/uterine fibroids
Abnormal placentation, especially placenta previa, can result in a decrease in the perfusion (blood supply) to the fetus. The presence of uterine fibroids may decrease perfusion by "stealing" blood away from the baby. Decreased perfusion results in decreased access to nutrients, thus resulting in lower birth weights.