So then why do their chests rise and fall in the ultrasound if they're not breathing?
They are "practicing." It is lung development.
Practice breathing in the womb is essential for lung development. learn now about, "when are babies lungs fully developed," the lung development stages,
birthingbalance.com
“Development of the lower respiratory tract begins on day 22 and continues to form the trachea, lungs, bronchi, and alveoli”
by 36 weeks, your baby’s lungs are formed enough to take their first breath
at 3-6 weeks of pregnancy:
At 3-6 weeks gestation, your baby’s lower respiratory tract begins to develop. A small pouch of tissue called
the lung bud is created from the front wall of the tube that will become your baby’s esophagus.
Over the next week, the single bud will split into two separate buds (one for each lung), and the trachea (windpipe) will start to form between them.
By the end of 6 weeks, your baby’s larynx, trachea, lungs, lobes of the lungs, and bronchopulmonary segments have formed! (Aka, the basic structures of the respiratory system).
from 5-18 weeks:
From 5-17 weeks, this stage of fetal lung development is primarily responsible for generating the bronchial tree. By the end of week 16, your baby will have their first 20 generations of the respiratory tree!
At 17 weeks of pregnancy, your baby’s lungs have developed their main bronchioles, cartilage, and smooth muscle. However, the respiratory bronchioles have not developed yet. Therefore, infants born at this stage are unable to facilitate gas exchange and are unable to survive [1].
At about 18 weeks, the smallest tubes (bronchioles) start to develop at the tips of the branches. At the end of these tiny tubes, respiratory sacs that eventually form the alveoli begin to appear.
The alveoli are where the lungs and the blood exchange oxygen and carbon dioxide during the process of breathing in and breathing out.
From 24-40 weeks:
From 24 weeks of pregnancy on (until your child is about 8 years old) these respiratory sacs grow and multiply, adding more surface area for the exchange of oxygen and carbon dioxide.
Starting at 24 weeks, they start making a substance called surfactant, which coats the alveoli and keeps them inflated when there’s not a lot of air in the lungs (when exhaling, for example).
Adequate surfactant production is not until about 35 weeks, so infants born at that time have a much higher likelihood of independent breathing and survival than those born at 24 weeks.
By 40 weeks, your baby has about 150 million alveoli in his lungs, ready to take over the job of breathing once he’s born. By the time your child is eight years old, he will have another 150 million, which is the same number as you!
What is Practice Breathing?
Practice breathing is important for lung development and preparing for life outside of the uterus. Practice breathing can start as early as 10 weeks.
Between 24-28 weeks of gestation, fetuses practice breathe about 14% of the time [3]. Then, by 34 to 40 weeks practice breathing increases to about 30% of the time [4].
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