C# Regex to match words from list

I have a following problem with my regular expression, I would like it to match caterpillar in the string "This is a caterpillar s tooth" but it matches cat. How can I change it?

        List<string> women = new List<string>()
        {
            "cat","caterpillar","tooth"
        };

        Regex rgx = new Regex(string.Join("|",women.ToArray()));


        MatchCollection mCol = rgx.Matches("This is a caterpillar s tooth");
        foreach (Match m in mCol)
        {
            //Displays 'cat' and 'tooth' - instead of 'caterpillar' and 'tooth'
            Console.WriteLine(m);
        }