While somewhat less common in the US as it is in other countries where Chinese Medicine/Acupuncture are more tightly integrated with Western Medicine, acupuncture is very often used to promote labor.  In the US we tend to perform treatment outside of a hospital setting which requires the use of "lighter" techniques so as to promote labor but not to strongly induce it as you would in a hospital setting.  It is effective both ways judging from clinical results.

In a hospital setting it is common to use electro acupuncture (EA) for labor induction as you can, in theory, create stronger stimulation of the acupuncture points.  Within this animal (rat) study a team of researchers from the School of Acupuncture-Moxibustion and Tuina at the Beijing University of Chinese Medicine looked at the best EA settings for labor promotion.

Two of the most common points for labor induction are LI 4 and SP 6.  This study used 108 female healthy Wistar rats which were divided into 8 groups including a non pregnant control group.  The treatment groups utilized various EA settings to determine the best outcome overall.  Treatment (with varying settings) was bilateral LI 4 for 20 minutes and then bilateral SP 6 for 5 minutes.

Compared with the control group all measures (amplitude, frequency, etc.) of uterine contractions were increased in all treatment groups.  Out of all the treatment groups the setting which created the most effective induction was EA with sparse-dense wave (2 Hz sparse wave and 50 Hz dense wave, alternately).