Have you ever wondered what determines the size of a newborn? Why do
some large parents have tiny children, while some petite parents
deliver hefty infants? What happens to these large, small or average
babies after they're born. Do large babies turn into large adults? Do
small babies stay small forever?
While prediction is always an inexact science (and obstetricians
admit they're frequently surprised by the size of a baby -- even after
ultrasounds), we consulted a variety of experts to help answer these
questions of size.
Which determines the size of a newborn: genetics or environment
(meaning the intrauterine environment, or what the fetus experiences in
the womb)?
Both. There's no easy answer here, say
obstetricians. Certainly the size of the baby's parents is a factor...
but only one factor. Some research even suggests that the weight of the
baby's mother at birth may be predictive of a baby's size... although
that's still unproven.
Of course, environment plays a powerful
role, too. A mother who smokes, abuses drugs, or doesn't maintain a
good diet during pregnancy, or has uncontrolled high blood pressure may
deliver a smaller-than-average baby. A woman with unrecognized or
uncontrolled diabetes may deliver a larger-than-average child.
Illnesses and medications during pregnancy may influence a baby's size,
too.
"Our predictions at estimating fetal size are only right
about 50 percent of the time," admits obstetrician/gynecologist Richard
W. Henderson, M.D., who practices in Wilmington, Delaware. "Even
ultrasound isn't accurate, so it's a lot of guesswork. And all of us
have been startled to deliver babies much larger or smaller than we'd
expected them to be."
What is considered normal or average size for a newborn?
Seven
and a half pounds is considered the average weight for a healthy
newborn, with variations on either side of a pound or a pound and half
still considered well within normal limits -- meaning anywhere between
six and nine pounds. Most newborns are between 19 and 21 inches long.
These
numbers refer to full-term infants. Babies that are born prematurely
(before thirty-seven weeks) or late (up to two weeks after their due
date) may fall on either side of these limits.
By the end of
the seventh month of pregnancy, the average baby weighs about 3 pounds
and is about 15 inches long. After the 34th or 35th week, babies gain
about one half pound a week until delivery. That means that a child
born at 36 weeks weighing five and a half pounds is right on target for
size and development, but a child who weighs five and a half pounds
after a full-term pregnancy is on the small side for its age.
On
the other hand, a pregnancy that is carried to 42 weeks -- two weeks
past due date -- may produce a larger-than-average baby. "But only if
the placenta is still functioning well," explains Dr. Henderson. If the
placenta has already started to break down after 40 weeks, the baby may
not grow much more until after birth.
Is it possible to predict from a baby's birth weight and length how big an adult he will be?
Absolutely
not. Any pediatrician can tell about the tiny preemies who grow into
basketball player-size men, or the giant babies who turn into
average-size adults. The reason pediatricians keep close tabs on a
baby's height and weight isn't to predict the future, but to make sure
the child is developing properly, whatever his initial -- and final --
size. Doctors are less concerned with the specific height and weight of
a child, and more concerned with the pattern of continued growth.
The
growth charts that pediatricians consult compare a child's size with
the general population. So a child who falls into the 50th percentile
for height and weight will probably continue to follow that growth
curve for his first year or so. But some children jump their curve;
they may start out life in the 50th percentile for size, but then grow
more quickly than average and jump to the 95th. That may be when genes
come into play, and the small child of large parents starts to grow
into his genetic heritage. Assuming that a child is in good health --
and his growth isn't compromised by illness or poor diet -- his final
size will have far more to do with his parents' size than his birth
weight.
A very rough guideline is that if you double a child's
height at age two you will have an approximation of his eventual adult
height. But there are many exceptions to this rule.
There's so much in the news now about the increasing
numbers of overweight children. Are chubby babies at risk of becoming
chubby adults? If my child is in the 95th percentile for weight, should
I worry?
Pediatricians are very concerned about the
incidence of overweight among American children. The Centers for
Disease Control is now encouraging doctors to look beyond height and
weight measurements, and measure children's BMI -- Body Mass Index.
This is a measurement that correlates height and weight, to give a
better idea of healthy size. Before, when pediatricians only consulted
height and weight charts, they might see that a child's weight was at
the 50th percentile for his age, and consider that perfectly healthy.
But what if the child was extremely tall? Or extremely short? The same
weight that was fine for an average height child could be too thin, or
too heavy, for another child, depending on his size.
The
formula for calculating BMI is a bit complex, and children's BMI
changes as they grow. From age two, BMI gradually drops until between
age 4 and 6, when it begins a steady climb to adulthood. (Think about
how toddlers thin out as they become more active, and then how older
children fill out more as they grow.)
Pediatricians can use
BMI numbers to determine which children are overweight... or at risk of
becoming overweight adults. For example, children whose BMI begins
climbing early -- before age 4 -- are at risk of weight problems. Also,
children with a BMI in the 95th percentile or higher are overweight,
while those with a BMI in the 85th percentile or higher are at risk of
overweight. Many doctors believe that children with a high BMI at age 8
are at risk of adult weight problems, and identifying these children
early can help them develop healthy eating and exercise habits.
Beth Weinhouse is a frequent contributor to Your Baby Today. She specializes in women's and children's health issues and lives in Oxford, Mississippi with her husband and 6-year-old son.
