from pubmed - these are just partial quotes from abstracts
Ann Med 1993 Apr;25(2):181-184
Contraception during lactation.
Saarikoski S
University Hospital of Kuopio, Department of Obstetrics and Gynaecology,
Finland.
"Combined oral contraceptives diminish milk production and therefore
combined oral contraceptives are not recommended during lactation. "

Baillieres Clin Obstet Gynaecol 1996 Apr;10(1):25-41
Post-partum contraception.
Kennedy KI
Family Health International, Research Triangle Park, NC 27709, USA.
"Combined oestrogen/progestin methods should be avoided by all women for 2-3
weeks to avoid elevating the risk of thromboembolism. Preparations
containing oestrogen should be avoided altogether during lactation because
they have been associated with a reduction in milk
production."

J Hum Lact 1996 Dec;12(4):315-318
Hormonal contraception and lactation.
Kelsey JJ
Cook County Hospital, Chicago, Illinois 60612, USA.
" Progestin only contraceptives do not appear to affect milk volume,
composition, or to cause deleterious effects in the infant. Ideally for
women who desire a form of contraception in addition to lactation-induced
amenorrhea, progestin-only methods should be started at six weeks postpartum
if the woman is fully breastfeeding. Since contraception protection is
provided by lactation amenorrhea, the six week delay will decrease infant
exposure to exogenous hormones and decrease the incidence of irregular
postpartum bleeding. Milk volume may decrease with the use of estrogen;
however, no detrimental effects have been shown
on infant growth or development. For women who are planning to gradually
wean their infant, use of COCs may provide an easier transition to
bottle-feeding. COCs should be used with caution by women who are not able
to obtain supplemental milk. A decrease in milk volume can lead to earlier
discontinuation of the hormonal contraceptive in an attempt to increase
milk quantity. Supplementation is often needed, and then the woman ovulates
again, possibly resulting in an unintended pregnancy."