So, the only effect noted for cannabis was a healthy increase in placental weight? Interesting!  And then when you add in these results.....

"Marijuana influences visual development" (Scoop) where cannabis PROTECTS against some of the negative effects of fetal alcohol syndrome.  

"Medical marijuana: a surprising solution to severe morning sickness" (Mothering). And as a woman, I can tell you it works!        

And in  “Mortality Within the First 2 Years in Infants Exposed to Cocaine, Opiate, or Cannabinoid During Gestation” (PubMed), they found “Within the first 2 years of life, 44 infants died: 26 were drug negative (15.7 deaths per 1000 live births) and 18 were drug positive (13.7 deaths per 1000 live births). The mortality rate among cocaine, opiate, or cannabinoid positive infants were 17.7, 18.4, and 8.9 per 1000 live births, respectively.”
  
Allow me make that even clearer, so there is absolutely no confusion-

No drugs at all babies- 15.7 deaths per 1000 live births
Cocaine-exposed babies – 17.7 deaths per 1000 live births
Opiate-exposed babies – 18.4 deaths per 1000 live births
Cannabis-exposed babies- 8.9 deaths per 1000 live births

An online article that goes into detail about this study is “Pregnant Women Smoking Pot Could Reduce Infant Mortality” (opposingviews).

However, smoking anything is bad for the baby, but cannabis can be "vaporized", eaten, or made into tinctures to avoid the hazards of smoking.