I found this:-
Drugs in pregnancy and lactation. Briggs, Freeman and Yaffe. Ed Williams and Wilkins, 1990.
Delta-9-tetrahydrocannabinol (TCH), the main active ingredient of marijuana, is excreted into breastmilk. Analysis of THC and two metabolites, 11-hydroxy-THC and 9-carboxy-THC, were conducted on the milk of two women who had been nursing for 7 and 8 months, respectively, and who smoked marijuana frequently (69). A THC concentration of 105 ng/ml, but no metabolises, was found in the milk of the woman smoking one pipe of marijuana daily. In the second woman, who smoked seven pipes/day, concentrations of THC, 11-hydroxy-THC, and 9-carboxy-THC were 340 ng/ml, 4 ng/ml, and none, respectively. The analysis was repeated in the second mother, approximately 1 hour after the last use of marijuana, using simultaneously obtained samples of milk and plasma. Concentrations (in ng/ml) of the active ingredient and metabolites in milk and plasma (ratios shown in parenthesis) were 60.3 and 7.2 (8.4), 1.1 and 2.5 (0.4), and 1.6 and 19 (0.08), respectively. The marked differences in THC found between the milk samples was thought to be due to the amount of marijuana smoked and the interval between smoking and sample collection. A total fecal sample from the infant yielded levels of 347 ng of THC, 67 ng of 11-hydroxy-THC, and 611 ng of 9-carboxy-THC. Due to the large concentration of metabolites, the authors interpreted this as evidence that the nursing infant was absorbing and metabolizing the THC from the milk. In spite of the evidence that the fat soluble THC was concentrated in breastmilk, both nursing infants were developing normally.
In animals, THC decreases the amount of milk produced by suppressing the production of prolactin and, possibly, by a direct action on the mammary glands (42) While data on this effect are not available in humans, maternal marijuana use does not seem grossly to affect the nursing infant. In 27 infants evaluated at 1 year of age, who were exposed to marijuana via the milk, compared to 35 nonexposed infants, no significant differences were found in terms of age at weaning, growth, and mental or motor development.
Although no adverse effects of marijuana exposure from breastmilk have been reported, follow-up of these infants is inadequate At the present time, the long term effects of this exposure are unknown and additional research to determine these effects, if any, is warranted. The American Academy of Pediatrics considers the use of marijuana during breastfeeding to be contraindicated.
Breastfeeding and drug exposure. CR Howard and RA Lawrence. Obstetric Gynecol Clin of North America 1998; 25(1): 195-217.
Animal studies indicate that THC reduces prolactin levels. Data on this effect, however, are not available in humans. Marihuana appears in human milk as THC, which is poorly absorbed in this form; however; with prolonged exposure, the compound may cause an infant to be lethargic and to feed less frequently and for shorter periods of time. In infants evaluated at 1 year of age, no differences were found in growth or mental or motor development when infants exposed to marihuana in breast milk were compared with non-exposed infants. Follow-up of these infants, however, has been limited, and long-term effects of marihuana exposure through breastmilk are unknown.
Maternal judgment may be impaired, and secondary behavioral changes may interfere with a mother's ability to care for her infant or to breastfeed adequately. As is true for cigarette smoking, passive exposure to marihuana poses additional risks to the infant. The American Academy of Pediatrics considers the use of marihuana during breastfeeding to be contraindicated.
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Dr. Thomas Hale (pediatrician and pharmacist) has been quoted as saying the marijuana tends to be sequestered in peripheral compartments, that THC does enter milk easily, but due to low concentration in the maternal plasma, the absolute dose to the infant is low. If the mom smokes occasionally, she should expect that only small levels will be transferred to the infant, probably subclinical amounts, which is why the studies show no developmental abnormalities. The infant will show a positive drug screen, but it is not known for how long; probably correlates with the maternal dose ingested.
There is difficulty with many studies as the mothers are using other drugs along with marijuana. So one can not ascribe any negative effects to any one particular drug.
A study described in Pediatrics 1994; 93:254-260, (Dreher MC, Nugent K and Hudgins R) compared the babies of Rastafarian mothers (smoking up to 5 joints a day) and non-smoking controls. Both groups of mothers were breastfeeding. After one month, the babies of the heavily smoking mothers scored better on the developmental testing done. The speculation was that the Rastafarian mothers had better social networking and support for being new mothers.
All this evidence goes against the mainstream belief that marijuana is bad.
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So say you were to use it to induce labour - you wouldn't have a breastmilk risk because your milk hadn't come in yet?
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