The paper outlines a methodology that allows us to determine whether couples' fertility is supply constrained based on the response they give to the subjective desired family size question. The central idea of the paper is that, when faced with the desired family size question, both constrained and unconstrained couples compare their demand for children with knowledge of their biological supply and unconstrained couples respond with a number while constrained couples respond with a qualitative response such as, "It is Up to God" (UTG), that essentially conveys the notion of demanding as many children as the supply function can yield. I then test this interpretation using data from Bangladesh. I find that controlling for demand side characteristics, positive supply shocks (birth of twins) lowers the probability of UTG response while negative supply shocks (wife's infertility) significantly raises the probability of UTG response. Based on the percentage of women giving the UTG response, it can be concluded that fertility of many couples in Bangladesh was constrained by supply.