In a recent article in The Journal Warfield1 says:
It occurred to me that if there was a specific ferment in the blood-serum which was elaborated to split up the products derived from the placenta, syncytium especially, there should be also in the blood-waste, products of the metabolic activity of the growing fetus. These substances had to leave the body by some route and the most logical route was the urinary excretion. These products should be peptones and amino-acids, they should dialyze out and one should then be able to find them by means of the ninhydrin reaction in the dialysate. It was found that actually such is the case.
In support of this theory, tests on seventeen cases of pregnancy are cited, all of them giving positive result; seven cases in the puerperium were tested with the result that "the eleventh day was the earliest to show a
JAMISON C. THE PRESENCE IN THE URINE OF DIALYZABLE PRODUCTS REACTING TO NINHYDRIN. JAMA. 1914;LXII(14):1084-1085. doi:10.1001/jama.1914.02560390024011