Tyler Smith is chiefly responsible for the notion of the "ovarian nisus" at the tenth month as the determining cause of labor. He was of the opinion that ovulation occurred periodically during pregnancy as in the unimpregnated state, and that the reflex irritation, consequent upon the maturation of the ovum corresponding to the tenth menstrual epoch, was sufficient to determine an increased afflux of blood to the uterus, and thus to provoke labor.
At once, M. Cazeaux made the pertinent inquiry, if labor is entirely dependent upon ovulation why does it usually occur at the tenth lunar month, and not at the seventh, eighth, or eleventh? At a more recent period, it has been demonstrated by dissection that menstruation never occurs during gestation, and that probably ovulation is suppressed. Still Tyler Smith's hypothesis receives prominent place in our text-books, and a widespread conviction of its truth is made manifest by
OVARIAN NISUS NOT THE DETERMINING CAUSE OF LABOR.. JAMA. 1890;XV(1):25–26. doi:10.1001/jama.1890.02410270041003