Philippa, It is documented that hormonal surges are higher during simultaneous stimulation of both nipples; so if a woman tends to get headaches when her hormones are fluctuating, it will be more noticeable when she nurses two babies at the same time. Prolactin, oxytocin, and vasopressin have all been accused, in the medical literature, of causing headaches, but the jury is definitely still out; in other words: nobody knows for sure what causes these breastfeeding-related headaches. (Email me directly for references.) Ginna ****************************************************************************** Ginna Wall, MN, RN, IBCLC Phone (206)548-6368 Lactation Services Coordinator Fax (206)548-4809 University of Washington Medical Center Mailbox 356153 1959 NE Pacific Street, Seattle WA 98195 ******************************************************************************