From Cannabis use while Breastfeeding, an article on drugs.com:
Although published data are limited, it appears that active components
of marijuana are excreted into breastmilk in small quantities. Data
are from random breastmilk screening rather than controlled studies
because of ethical considerations in administering marijuana to
nursing mothers. Concern has been expressed regarding marijuana's
possible effects on neurotransmitters, nervous system development and
endocannabinoid-related functions.1[2] One long-term study found
that daily or near daily use might retard the breastfed infant's motor
development, but not growth or intellectual development.[3] This and
another study[4] found that occasional maternal marijuana use during
breastfeeding did not have any discernable effects on breastfed
infants, but the studies were inadequate to rule out all long-term
harm. Although marijuana can affect serum prolactin variably, it
appears not to adversely affect the duration of lactation. Other
factors to consider are the possibility of positive urine tests in
breastfed infants, which might have legal implications, and the
possibility of other harmful contaminants in street drugs.
Marijuana use should be minimized or avoided by nursing mothers
because it may impair their judgment and child care abilities. Some
evidence indicates that paternal marijuana use increases the risk of
sudden infant death syndrome in breastfed infants. Marijuana should
not be smoked by anyone in the vicinity of infants because the infants
may be exposed by inhaling the smoke. Because breastfeeding can
mitigate some of the effects of smoking and little evidence of serious
infant harm has been seen, it appears preferable to encourage mothers
who use marijuana to continue breastfeeding while minimizing infant
exposure to marijuana smoke and reducing marijuana use.[5]
So: It you want a final answer, it is maybe.
References:
Schuel H, Burkman LJ, Lippes J et al. N-acylethanolamines in human reproductive fluids. Chem Phys Lipids. 2002;121:211-27. PMID: 12505702
Fernandez-Ruiz J, Gomez M, Hernandez M et al. Cannabinoids and gene expression during brain development. Neurotox Res. 2004;6:389-401. PMID: 15545023
Astley SJ, Little RE. Maternal marijuana use during lactation and infant development at one year. Neurotoxicol Teratol. 1990;12:161-8. PMID: 2333069
Tennes K, Avitable N, Blackard C et al. Marijuana: prenatal and postnatal exposure in the human. NIDA Res Monogr. 1985;59:48-60. PMID: 3929132
Hill M, Reed K. Pregnancy, breast-feeding, and marijuana: A review article. Obstet Gynecol Surv. 2013;68:710-8. PMID: 25101905