How can a fetus grow upside down and not get blood rushing to the head?

Answer 1

A fetus actually stays more vertical with head up toward the ribcage, or stays horizontal across (within) the uterus throughout the 9 months of gestation. Many women complain of feeling the baby kicking the mother's side(s) and some moms say they feel their fetus doing a roll or flip as they stretch and move arms and legs.

In the weeks before a woman's due date for delivery, the fetus begins to turn to the "head down" position, except in breach cases when the fetus fails to turn head down. Gravity and pre-labor uterine contractions (Braxton-Hicks) help move the baby further down. The head must enter the pelvic opening long before true labor begins (doctors might say the head is engaged or has dropped, meaning the head is sitting within the opening of the pelvic bones). As the mother continues her daily routine, and with the aide of uterine muscles influenced by drops in certain hormones, the baby continually pushes against the inside of the cervix, helping it to "efface" or "soften".

You've asked why the fetus doesn't experience blood "rushing to its head" because of the upside-down position. First, the movement downward is slow, over days to weeks, so the baby adjusts to the "head-down" bloodflow, much like astronaunts become acclimated to weightlessness over time. Second, a normal fetal and neonate (just after birth) and the infant's heart rate is much higher than a "normal" for an adult. While adults have a normal heart rate of about 80 (give or take 10 beats), a fetal to infant's heart rate is normally 140 to 160. This keeps the fetus well-perfused (i.e. an adequate amount of circulating blood) to the brain and all extremeties and vital organs, regardless of the position within the womb.

There can be complications during the 3rd trimester or during labor/delivery that can severely impede fetal blood flow, but normally women are well-monitored so that the majority of complications are found quickly and corrected quickly.