I don't honestly know WHY it works, but it (ganja) has been used in India for centuries as an aid to stimulate contractions. Here's a little article about the medicinal use of ganja http://www.henriettesherbal.com/eclectic/kings/cannabis.html
In Jamaica, just about every household, even the most conservative, has a jar of ganja tincture tucked away somewhere, we used it for everything from upset stomach, teething or colicky babies, painful periods, headaches, cold and flu symptoms, fevers, labour analgesic, uterine contraction stimulant, you name it! We can thank our East Indian population for adding this contribution to our home remedy reportoire!
To make, it's typically 1 part ganja to 4 parts alcohol, we used overproof white rum, 110 proof. Put the clean, dry ganja is the bottom of a jar and fill to the brim with overproof white rum. (any alcohol will work, as long as it's over 70 proof) cover tightly and keep in a cool, dark place. It takes about a month, but it will turn a rich, dark green. When you make it specifically for women's complaints, you make it on the new moon, keeping it in the dark during the day and setting it in a window to absorb the moon's energy at night. It's ready at the next new moon. Add 10-30 drops to a cup of hot tea, depending on the strength of the ganja used. We usually don't use high grade for this, only the simple "bush" weed. You also want to make sure it was naturally and non-chemically grown, no hydro, no fertilizers.
I'm pretty sure it contans THC!