Estrus synchronization is necessary to manage batches of sows. It is usually achieved by using synthetic progestagens, but their use contaminates the environment and is forbidden in organic breeding. The aim of this prospective study was to test natural substitutes. Phytoprogestogens concentrations have been measured in red clover (kaempferol and apigenin) and yam (diosgenin). Gilts received a feed ration containing red clover or yam for 18 days, starting 10 days after estrus (estrus 1). Control groups received a classic feed ration with altrenogest® for 18 days starting 10 days after estrus 1 (synthetic synchronization) or without synchronization (3 gilts per group for yam, 4 for red clover). Gilts were subjected to daily standing estrus detection and sampling to analyze progesterone concentration in blood and saliva. Per day, gilts consumed a mean of 2 kg of red clover, containing 0.17 g kaempferol and 0.03 g apigenin/kg dry matter, or 1.1 kg of yam, containing 0.3 g of diosgenin/kg dry matter. In our conditions, consumption of phytoprogestogens had no effect on the interval between estrus 1 and estrus 2. Plasma progesterone concentrations dropped sharply between 15 and 20 days after estrus 1. Saliva progesterone from control, clover and yam groups decreased between 15 and 20 days after estrus 1, but the decrease was smaller than that of plasma progesterone. In our conditions, consumption of phytoprogestogens had no effect on progesterone concentrations in saliva or blood. However, the concentrations of phytoprogestogens in clover and yam were low, so varieties with higher concentrations should be tested.