Gestational Size During Pregnancy May Affect Childhood IQ, Study Finds

Key Takeaways

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

As early as your first prenatal appointment, your baby’s growth will be monitored. Every inch of your growing bundle of joy will be compared to other babies at the same stage of pregnancy—from the 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 ideal to give birth to a smaller baby. However, 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 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 (SGA) 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, lower socioeconomic status, associated with a 14-point lower IQ, and finally, poor parent-infant attachment, which was associated with a 10-point lower IQ.

What Causes a Baby to Be Small for Gestational Age? 

A variety of factors can cause a baby to be small for their gestational age. Also, there is a healthy range of weights and sizes that a baby can be that increases as the pregnancy progresses. That’s why some babies are born weighing 6 pounds while others might be 9 or more pounds at 40 weeks. This is normal, but some babies do not grow as expected.

It’s when a baby is below the healthy weight and size range for their specific gestational age that a baby is designated to be SGA.

It’s important to understand exactly what the term means, says Ron Caplan, MD, an OB-GYN and Clinical Associate Professor Emeritus at the Weill Medical College of Cornell University, in Ithaca, New York.

Ron Caplan, MD

The small for gestational age designation is not the same as low birth weight. SGA means that the fetus in utero is small for the stage of pregnancy, 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 a preemie 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, says Leann Poston, MD, a pediatrician and medical writer.

Chronic Maternal Health Issues

According to Dr. Poston, “If a mother has heart or lung disease, it can result in decreased blood and oxygen supply to the placenta.”


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 Dr. Poston.

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

Some elements of fetal growth and development are completely beyond your control, however, regular prenatal care and following your OB-GYN’s suggestions can improve the odds that your baby will develop and grow at the optimal rate. Ultimately, the best thing you can do is to get (and stay!) as healthy as possible during your pregnancy (and ideally beforehand).

Ron Caplan, MD

Optimal 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 take your 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 Dr. 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. Talk to your doctor if you have any questions or concerns about your baby’s growth and development.

Verywell Family uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. Marlow N. Understanding the effects of being small for gestational age at birth. BJOG: Int J Obstet Gy. 2020;127(13):1607-1607. doi:10.1111/1471-0528.16350
  2. Eves R, Mendonça M, Bartmann P, Wolke D. Small for gestational age ‐ Cognitive Performance from Infancy to Adulthood: An Observational Study. BJOG: An International Journal of Obstetrics & Gynaecology. 2020. doi:10.1111/1471-0528.16341
  3. Taylor RM, Fealy SM, Bisquera A, et al. Effects of nutritional interventions during pregnancy on infant and child cognitive outcomes: a systematic review and meta-analysis. Nutrients. 2017;9(11):1265. doi:10.3390/nu9111265

Additional Reading

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

By Christin Perry

Christin Perry is a freelance writer and editor. Her work has been published in The Bump, The Knot, Scary Mommy, LittleThings, Parents, Qeepsake, and more. She has experience writing email marketing campaigns, website copy, and SEO-optimized content. Christin is also a mom of three.

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