A full-term pregnancy lasts 40 weeks from the start of the last menstrual period. Delivery at any time after 37 weeks is considered a full-term delivery. While most babies fall within certain weight parameters, fetal weight varies considerably at 37 weeks from baby to baby, depending on a number of maternal and fetal factors.
The average newborn weighs 7.5 lbs., according to the What to Expect website, with 95 percent of all newborns falling into a range of 5 to 10 lbs. at birth. A fetus normally gains about 8 oz. per week in the last month of pregnancy, according to MayoClinic.com, so the average infant would weigh 6 lbs. at 37 weeks, with a range of 3.5 lbs. to 8.5 lbs.
A pregnant woman's lifestyle and other factors can influence a newborn's weight at 37 weeks. Smoking, using drugs or alcohol, or malnutrition during pregnancy can cause a baby to weigh less than normal. Mothers with high blood pressure or heart disease can also have smaller-than-normal babies, while diabetic mothers may have larger-than-normal babies. Placental problems during pregnancy restrict fetal nutrition and can cause smaller-than-normal babies. Heredity also plays a part in infant size; larger people tend to have larger babies.
Babies born in multiple pregnancies, especially higher order multiple births with three or more fetuses, weigh less than normal throughout the pregnancy and often deliver before 37 weeks. Genetic abnormalities such as chromosomal abnormalities or heart disease in the fetus can also result in lower than normal weight at 37 weeks.
It's impossible to determine a fetal weight just by palpating the abdomen, because maternal weight and the amount of amniotic fluid surrounding the baby also affect abdominal size. Fetal ultrasound, while helpful, tends to underestimate large babies and overestimate small ones, according to a 2006 study reported in "The New Zealand Medical Journal."
There's no truly accurate way to determine an infant's weight at 37 weeks until he emerges from the womb and is placed on a scale in the delivery room. Many factors influence fetal weight and many babies deviate from the average, for a variety of reasons.