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How Big Will My Baby Be?

Are you pregnant and concerned about your ability to birth a "big baby?" In late pregnancy, ultrasound and other methods are used to try to estimate a baby's size at birth but these are not a perfect science and can sometimes leave you with more questions than answers and fears about your ability to birth you baby vaginally. This can be especially anxiety inducing if you've been told that your baby may be on the larger end of the spectrum.

But how accurate are these estimates, and how does the information affect choices surrounding birth?

The medical community generally defines a "large baby" as over 8 lbs 13oz. You shouldn't worry too much, statistically speaking, fewer than 8% of babies born in the US meet this definition.

A new study surveying 1,960 new moms found that four out of five of the mothers who were warned that they might have a big baby did not, in fact, give birth to a large infant. However, those mothers were almost twice as likely to have medical interventions such as labor induction, and almost twice as likely to have a planned cesarian birth.

A 2012 study showed that simply being told the estimated birth weight of a baby caused a significant increase in the percentage of cesarian births when all other variables were controlled.

The American College of Obstetricians and Gynecologists (ACOG) has published guidelines stating that suspecting a large baby is only "rarely" an indication for a planned cesarian birth. These guidelines say that doctors should only offer a cesarian to women with a baby predicted to weigh over 11 lbs at birth.

As we learn more about the role of womens' past experience, fears, and preconceptions on the processes of labor and birth, it's important to recognize the effect of the large volume of information that women receive during pregnancy. While technological advances and the ability to collect more data can be helpful, anxiety regarding a baby's size may influence how an expecting mother approaches vaginal birth or considers elective cesarian birth.

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Liz Miracle Headshot
Dr. Liz Miracle, PT, MSPT, WCS

Liz Miracle is Origin's Bay Area Clinical Director. She graduated from Texas Woman's University School of Physical Therapy and is now the owner of Miracle Physical Therapy in San Francisco, CA.

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