Month 9

But the most fascinating discovery? The foods a mother eats today may influence which foods her child enjoys years later. Flavors from a mother’s diet pass into amniotic fluid and breastmilk, giving babies early exposure to family foods.¹⁵ As a result, kids who are exposed to fruits, vegetables, and even bold flavors like broccoli or sour apple juice inside the womb will enjoy similar foods after birth.^{16 17}

From familiar stories to sweet anise, early exposure helps shape preferences after birth, showing that learning begins long before the first snuggle.

In the final month before birth, the unborn baby stays remarkably active. Each day, he swallows one to two cups of amniotic fluid and produces more than two cups of urine.²⁷ Although the amniotic fluid provides about 40 calories daily,²⁸ the placenta delivers most of the nutrients and oxygen that support his rapid growth. As a result, he gains nearly half a pound per week on average.²⁹ To keep the amniotic fluid clean, the amniotic fluid circulates rapidly. In fact, the water content of the amniotic fluid is completely replaced every three hours!³⁰

Although only about the size of a walnut, the newborn heart pumps roughly 500 gallons of blood each day, which would likely fill a large hot tub!³¹ For comparison, an adult heart pumps three times that amount: about 1,500 gallons of blood each day. After birth, heart cells stop multiplying and instead grow bigger. Just one month after birth, the baby has nearly all the heart muscle cells he will ever have.³²

By 38 weeks, mature alveoli start forming in the lungs. About 95% of alveoli develop after birth. Most of the gas exchange area is created by subdividing existing air space to create new alveoli. At birth, newborns have about 150 million alveoli, approximately half of the alveoli found in an adult. Many of these early alveoli are still immature. Children keep growing more alveoli until about age 8, reaching a total of about 300 million.³⁴ New research shows that a few new alveoli continue to grow up to 21 years old!³⁵

To support rapid growth, fetal blood circulates at more than twice the rate seen in newborns.³⁸ Furthermore, the fetal heart beats quickly, between 110 and 170 beats per minute, and sends over 40% of its output to the placenta. The left side of the heart delivers oxygenated blood to the brain and heart muscle, ensuring major organs receive what they need to develop properly. At birth, the newborn heart rate decreases to about 100–120 beats per minute. Breathing outside a mother’s womb transforms the baby’s circulation entirely.³⁹ This elegant, transient system allows the unborn child to thrive in the womb until birth.

Although they fit inside the baby’s small abdomen, the intestines have already grown remarkably long by 9 months gestation. The small intestine stretches about 9 feet—roughly the height of a basketball hoop—while the large intestine stretches about 2 feet.⁴⁰ In adults, these organs grow to about 22 feet and 5 feet, respectively.^{41 42}

In adults, lots of healthy bacteria inhabit the intestines. These bacteria help digest food, synthesize vitamins, provide immune system support and influence mood. Researchers have found that the same bacteria that aid in digestion and immunity only enter the intestines of healthy babies during and after birth.⁴⁵