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
