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  Pregnancy eMagazine

How big is my baby? (35 to 40 weeks)

How fast is my baby growing?

Babies grow quickly in the last few weeks of pregnancy. The average baby adds about a pound and a half between 35 and 40 weeks of gestation. This rapid growth slows after 41 weeks of gestation as the baby begins to outgrow the placenta.

The average birth weight at 40 weeks gestation is 3,400 grams (7 pounds and 8 ounces) at sea level and 3,200 grams (7 pounds and 1 ounce) at an elevation of 5,000 feet.

For more details on the growth of your baby, see Third trimester: What's happening to your baby.

What determines how fast my baby grows before birth?

A mother's nutrition, placental function, general health and physical size all influence the growth of her baby. For example, small women are twice as likely as large women to have small babies. Small women usually have a small birth canal, so small babies may be an advantage to these mothers and their babies. Within the range of normal birth weights (for full-term infants: 5 lbs. 9 oz. to 9 lbs. 1 oz.) size does not carry a long-term advantage.

What is normal?

Normal-sized babies are those whose birth weights are between the 10th and 90th percentiles. These babies are called appropriate for gestational age or AGA. AGA babies have the fewest problems during labor, delivery and immediately after birth.

Children falling below the 10th percentile (smaller than 90 percent of other infants) are considered small for gestational age (SGA). Children who are above the 90th percentile (larger than 90 percent of other infants) are considered large for gestational age (LGA).

How will I know if my baby is growing normally?

The doctor or midwife assesses your baby's growth and size at each prenatal visit by measuring your abdomen. If there is a question about your baby's growth, your doctor or midwife may order an ultrasound examination, which gives more accurate estimates of weight than the physical exam.

What if my baby is appropriate gestational age (AGA)?

Appropriate gestational age (AGA) infants have the best long-term outcome. Large for gestational age (LGA) babies have a higher risk of injury during birth, low blood sugar after birth and other problems when compared to AGA babies. Small for gestational age (SGA) babies also are more likely to have low blood sugar and other problems when compared to AGA babies.

Birth weight does not usually predict later growth. In the first year of life, many LGA babies will slow their growth and many SGA babies will increase their growth to get closer to the average weight for their age.

What if my baby is small for gestational age (SGA)?

Some babies grow more slowly than the "average" baby resulting in an SGA baby. SGA is commonly defined as being smaller than 90 percent of all similarly aged babies. Using this definition, 10 percent of all babies will be SGA. But only a portion of that 10 percent will have problems.

Your doctor or midwife may use other names for your SGA baby:

  • intrauterine growth retardation (This term is now out of favor because it incorrectly implies mental retardation, which is not usually present.)
  • intrauterine growth restriction
  • small for dates
  • dysmature
  • light for dates

The implications of being SGA depend on when in your pregnancy it begins, how small your infant is, the amount of head growth restriction and the cause of the growth restriction. To learn more, see slowed growth of baby.

What if my baby is large for gestational age (LGA)?

Babies who are larger than 90 percent of similarly aged babies are called large for gestational age (LGA). You might think being big would confer advantages to the fetus and newborn baby. However, just the opposite is true. LGA babies have a higher chance of birth injury, low blood sugar, elevated red blood cell count, and immature lungs than appropriate gestational age (AGA) infants.

It was once thought that these infants' weight and size became normal later in life and that there were no long-term consequences to being LGA. New research casts doubt on this assumption. LGA infants born to mothers with gestational diabetes probably have a higher risk of developing obesity in childhood and diabetes later in life.

A baby may be large for gestational age due to factors like these:

  • maternal diabetes (including gestational diabetes)
  • maternal obesity
  • enhanced growth potential (Some babies are just big.)
  • rare fetal abnormalities (certain syndromes)

The severity of the problem associated with being LGA depends on how large the baby is and why the excess growth has occurred. Mothers with diabetes mellitus can reduce their risk and their baby's risk of having problems by strictly adhering to their diet and controlling their blood sugar.

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Source: Children's Hospitals and Clinics of Minnesota

First published: 05/11/2001
Last updated: 10/14/2007

Reviewed by: Michael Slama, MD, Allina Health Mercy Women's Health Clinic