Gestational Size During Pregnancy May Affect Childhood IQ, Study Finds

Adult holding newborn baby hand

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Key Takeaways

  • A study performed in the UK found that babies that were designated as small for gestational age during pregnancy had slightly lower IQs during childhood.
  • The gap in IQ seemed to close as these babies approached adulthood.
  • Small for gestational age is a designation given to a fetus that's in the 10th percentile or lower.

As early as your first prenatal appointment, your baby’s growth will be a hot topic! Every inch of your growing bundle of joy will be compared to other babies at the same stage of pregnancy—from head circumference to leg bones, right down to the nasal bone.

These all-important measurements will determine your baby’s growth percentiles, which can help your doctor learn important information pertaining to your baby’s health. 

During pregnancy, your baby’s gestational age will be the benchmark used to determine whether your baby is growing properly and hitting milestones necessary to become a healthy, full-term infant when birth occurs around 40 weeks. 

When you think about the mechanics of childbirth, it can seem tempting to give birth to a small baby. But that’s only true to a certain extent: babies that measure in the 10th percentile or below for gestational age can be at risk of cognitive and health issues at birth. 

In fact, a study conducted in the British Journal of Obstetrics and Gynaecology found that babies born small for gestational age, or SGA, had lower IQs than those born at normal birth weights.  

Study Results

The study, which was conducted on 401 infants (both neonates and newborns) determined that the IQ of babies born small for gestational age was nearly 8 points lower than their counterparts of normal weight. Despite this initial cause for concern, however, researchers learned that the differences in IQ somewhat resolved as the babies approached adulthood. 

In addition to examining SGA babies, the study also looked at other common factors that are known to affect a baby’s IQ: preterm birth, associated with a 16-point lower IQ, low socioeconomic status, associated with a 14-point lower IQ, and finally, poor parent-infant relationship, which was associated with a 10-point lower IQ. 

What Causes a Baby to Be Small for Gestational Age? 

What are some factors that can cause a baby to be small for gestational age? First, it’s important to understand exactly what the term means, says Ron Caplan, MD, an obstetrician gynecologist and Clinical Associate Professor Emeritus at the Weill Medical College of Cornell University.

Ron Caplan, MD

The small for gestational age designation is not the same as low birth weight. SGA implies that the fetus in utero is small for the stage of pregnancy the mom is in, whereas low birth weight is simply a baby that weighs less than a certain amount at birth. 

— Ron Caplan, MD

In order to better understand the difference, think of a premature baby: since it was born early, it’ll have a low birth weight, but that weight could be totally appropriate for the baby’s gestational age. In that case, the baby wouldn’t be considered small for gestational age.

When a fetus is determined to be small for gestational age, there’s often a contributing factor that can be pinpointed. The fetus may not be getting adequate nutrition, or have enough room to grow for one reason or another. Here are some common factors that can cause a fetus to be small for gestational age:

  • Congenital infection: Most illnesses you pick up during pregnancy won’t harm your growing baby. But there are a few, such as cytomegalovirus, rubella virus, and Toxoplasma gondii, that can lead to issues with growth, Poston says.
  • Chronic maternal health issues: According to Leann Poston, MD, “If a mother has heart or lung disease, it can result in decreased blood and oxygen supply to the placenta.”
  • Preeclampsia: A study in the American Journal of Obstetrics and Gynecology found that “compared with healthy first-time pregnancies, a history of preeclampsia was associated with a significantly increased odds for a small-for-gestational-age infant, even if recurrent preeclampsia did not occur.”
  • Pregnant with multiples: When you’re pregnant with twins, triplets, or even more, there are multiple babies gestating who all share a placental blood supply and uterine space. This can lead to one of the babies being small for gestational age, says Poston.
  • Maternal alcohol and/or drug use: Drinking, smoking, and using drugs at any stage during pregnancy can cause reduced blood flow and can lead to a fetus being small for gestational age. 

Positive Impacts on Fetal Cognitive Development

Hoping to give your baby’s IQ a boost right from the start? The best thing you can do to give your little Einstein the best start is to get (and stay!) as healthy as possible during your pregnancy (or ideally beforehand).

Ron Caplan, MD

Good fetal development—including cognitive development—is greatly influenced by the intrauterine environment. Start prenatal care as soon as you find out you’re pregnant, eat a nutritious diet, follow your doctor’s guidance for exercise, and get plenty of rest.

— Ron Caplan, MD

And don’t forget those prenatal vitamins! Multiple studies have confirmed that taking a prenatal with DHA during pregnancy can have a beneficial impact on your baby’s IQ at birth and beyond. 

In addition to all these “to-dos” during pregnancy that may help boost your baby’s IQ, there’s an equally long list of things to avoid. According to Poston, “the safest thing to do is assume all drug use, smoking, chemical exposures, alcohol use, malnutrition, and lack of prenatal vitamins could adversely impact fetal cognitive development.” 

What This Means For You

If your baby is only slightly small for its gestational age, there’s a good chance you have nothing to worry about. The study cited in this article focused on fetuses that were classified as below the 10th percentile for growth. Chances are, your little one will start growing like a weed before you know it.

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  1. Eves R, Mendonça M, Bartmann P, Wolke D. Small for gestational age ‐ Cognitive Performance from Infancy to Adulthood: An Observational StudyBJOG: An International Journal of Obstetrics & Gynaecology. 2020. doi:10.1111/1471-0528.16341

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