Birth-control pills definitely affect hormones, and there are many conflicting opinions about their effect on hair loss or hair regrowth. Regrettably, there are few scientific studies on female pattern baldness, and even fewer when it comes to the effect of birth-control pills on this condition. The small amount of research that does exist shows that some birth-control pills have more testosterone-like activity, which can possibly promote hair loss by increasing the likelihood of testosterone being converted to DHT. That would cause a number of hair follicles to lapse into the telogen phase (shedding) and then not begin the anagen phase (growth) again. Further, the presence of testosterone can increase secondary male sex characteristics such as facial hair growth. It can also increase the likelihood of acne, because acne is frequently caused by androgen activity involving testosterone and DHT, the same hormones that trigger hair loss.
Many birth-control pills also contain minimal amounts of testosterone, or have antiandrogenic properties (meaning they inhibit testosterone). These formulas can therefore reduce hair loss and may actually help hair growth on the head, while also reducing the risk of acne. When birth-control pills contain estrogen, they can help reduce hair loss because estrogen makes hair stay on the head longer.
Because there are so many other complicated and significant health issues related to taking birth-control pills, hair loss and hair growth should not be the primary reason for taking them. It is essential for you to discuss all the pros and cons of these drugs at length with your physician.
(Sources: Contraception, May 2008, pages 337-343; Current Drug Safety, August 2006, pages 301-305; Obstetrics and Gynecology, May 2003, pages 995-1007; Drugs, 2003, volume 63, issue 5, pages 463-492; and American Journal of Clinical Dermatology, 2002, volume 3, issue 8, pages 571-578.)