Saffron: Should only be avoided in large doses (if you can even afford large doses!). Large doses can cause contractions in the smooth muscle of the uterus, stimulate menstural flow, and may induce abortion. An interesting note; ingested at high enough doses, saffron is considered lethal.
Sage: A uterine stimulant and should be used sparingly only as a culinary spice during pregnancy. Large amounts of sage should be avoided by breastfeeding mothers because it is known to decrease milk supply.
Sarsaparilla (root): According to the German Commission E monograph, sarsaparilla root may cause stomach irritation and temporary kidney irritation. It contains saponins, which have laxative and diuretic properties. Most sarsaparilla *drinks* - that I know of - only contain an imitation or natural sarsaparilla flavoring, which should not be strong enough to cause symptoms. Consult your healthcare provider if you are concerned.
Sassafras
Saw Palmetto
Senna
Sheep Sorrel
Shepherd’s Purse
Skunk Cabbage
Solomon’s Seal Root
Spikenard Root
St. John’s Wort
Suma Root: The safety of suma root has not been established for use during pregnancy or breast-feeding. According to published data, what little research has been done on the effects of suma root has been completed only in test tubes or with animals. You may actually find information indicating that suma root consumpton is safe for pregnant/nursing women but due to the lack of sufficient medical study, many herbalists, naturopaths, and medical professionals believe suma should be used with extreme caution in women who are pregnant/nursing.
Sumac: Has been shown to stimulate uterine contractions. Tansy Thyme: A uterine stimulant (can cause contractions). Normal culinary use is considered safe, but therapeutic or medicinal doses of thyme during pregnancy/lactation should be avoided.
Tonka Bean
Tribulus Fruit (Puncture Vine)
Turkey Rhubarb
Turmeric