Weeks 21 & 22

Breathing outside the womb is an amazing and complex process. This is why the unborn baby consistently practices breathing starting at gestational week 13!¹⁷ Fetal breathing movements push amniotic fluid into and out of the developing lungs. At the end of the bronchioles, the moving amniotic fluid helps inflate the tips into future alveoli, where gas exchange will occur. At the same time, these mechanical forces guide lung cells to differentiate into two critical types: type I cells, which exchange gases, and type II cells, which produce surfactant, a slippery substance that prevents alveoli from sticking shut when the baby breathes out.¹⁸

In month 5 of pregnancy, the unborn child begins to detect and respond to sounds outside the womb. In one study, a fetus at just 19 weeks gestation moved in response to a loud, low-pitched tone, similar to a lawn mower.¹⁹ Unborn babies begin responding to sound at different weeks of pregnancy, much like infants reach milestones such as crawling at different ages. By 21 to 22 weeks, about half of babies had a heart rate acceleration when exposed to a high-pitched sound like the beep of a microwave, showing their hearing is starting to work.²⁰

Starting at 5 months gestation, the fetus’s movements and breathing follow a daily cycle, called a circadian rhythm.²¹ Interestingly, fetal activity, especially breathing movements, increases about two hours after the mother eats, and then starts to decrease.²² The baby’s heart rate follows a daily pattern as well.²³

Fewer than 1% of babies are born extremely premature—before 28 weeks—when the brain is still developing rapidly, with neurons migrating, white-matter pathways forming, and circuits for movement, learning, and vision taking shape. When a baby is born very early, this development continues outside the womb under far more stressful conditions, making the immature brain vulnerable to injury and disrupted growth. As a result, extremely preterm infants have higher risks of long-term challenges affecting cognition, motor skills, vision, and learning. Advances in neonatal care—including improved respiratory support, protective brain monitoring, and therapies that promote healthy brain growth—are helping more of these tiny patients survive and develop with better outcomes than ever before.³⁰

In females, the number of future eggs in the ovary peaks at 7 million around 5 months. After this, the ovary essentially stops creating future egg cells, and most of these cells die before birth. In fact, only between 600,000 and 800,000 future egg cells remain by birth. Only 40,000 of these cells will remain by the start of puberty, and less than 500 of these cells will be released in ovulation.⁴⁰