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What predisposes fetuses to orient themselves in the vertex presentation?

My doctor explained it with gravity but I am not convinced.

The head is the heaviest, fair enough. The gravitational pull experienced by a body is proportional to its mass. Hence the pull on the heavier head is greater.

While true, I don't see any explanatory value. Given it feels greater gravity on it's head, this just means the head feels heavier.

The fetus still orients through motor movements. What propels it to typically orient in the vertex presentation?

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  • I don't blame you for not believing the response you were given. It's simply wrong. @Timur Shtatland is correct. Simply put, the uterus can expand more (so is larger) at the top, where there are no bones, than the bottom, which is surrounded by ligaments and the bony pelvis. As the bottom half of a baby (buttocks and legs) is larger than the shoulders and arms, and the head smaller yet than those, the head-first presentation is pysiological, and most common. Mar 24 at 16:19
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Vertex presentation is caused by the shape of the uterus, rather than (directly) caused by gravity.

REFERENCES:

In vertex presentations the head of the fetus most commonly faces to the right and slightly to the rear. This position is said to be the most usual one because the fetus is thus best accommodated to the shape of the uterus.

Presentation | childbirth | Britannica: https://www.britannica.com/science/presentation#ref55860

The piriform (pear-shaped) morphology of the uterus has been given as the major cause for the finding that most singletons favor the cephalic presentation at term.

Cephalic presentation - Wikipedia: https://en.wikipedia.org/wiki/Cephalic_presentation

In about 95% of deliveries, the part of the fetus which arrives first at the mother’s pelvic brim is the highest part of the fetal head, which is called the vertex. This presentation is called the vertex presentation. Notice that the baby’s chin is tucked down towards its chest, so that the vertex is the leading part entering the mother’s pelvis. The baby’s head is said to be ‘well-flexed’ in this position.

During early pregnancy, the baby is the other way up — with its bottom pointing down towards the mother’s cervix — which is called the breech presentation. This is because during its early development, the head of the fetus is bigger than its buttocks; so in the majority of cases, the head occupies the widest cavity, i.e. the fundus (rounded top) of the uterus. As the fetus grows larger, the buttocks become bigger than the head and the baby spontaneously reverses its position, so its buttocks occupy the fundus. In short, in early pregnancy, the majority of fetuses are in the breech presentation and later in pregnancy most of them make a spontaneous transition to the vertex presentation.

The Open University: Labour and Delivery Care Module: 8. Abnormal Presentations and Multiple Pregnancies: https://www.open.edu/openlearncreate/mod/oucontent/view.php?id=276&printable=1

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