To be honest, I would advise all pregnant women, with the exception of those with severe morning sickness, to stay away from mary jane, during the first three months of pregnancy, where the development of organs and human functions are at their critical stage. Large amounts of alcohol (a glass of wine once every week or two is probably okay), opiates, nicotine, prescription drugs with pregnancy contraindications, cocaine, LSD, etc, should ALWAYS be avoided, first three months, up until child is 18 YEARS old. However, I think that smoking a joint for labor is FAR less likely to do damage or cause adverse reactions than the standard demerol, stadol, morphine, or duramorph/epimorph (both used in different epidurals/spinal blocks). There is a reason doctors will not give you an epidural before a certian stage in labor ( 3 or 4 cm dilated) or after a certian stage in labor (I think the cutoff is 5 or 6 cm dilated). That can be a VERY short time frame for some women, especially those that are not first-time moms, where labor can progress VERY quickly.
To sum it up, don't use MJ unless you absolutely need it in pregnancy/during labor. And if you do use it, PLEASE get it from a reputable source that does not lace it with additives or chemicals. On a side note, I do not smoke mary, so I do not know if you can get an additive/chemical free, clean joint from anyone but a medical professional, so I am unsure of how natural a joint you can get off the "streets" per se. I would hope that there are some dealers out there that provide just good old Mary Jane, without lye, tar, or whatever else some people add to the herb. I think that some of the chemicals and additives in mary are way more harmful than the herb itself and are probably what gives the herb a bad name. Please, please, if you are gonna toke it, choose your source WISELY. Some people will "lace" mary with heroin, cocaine, etc, in an effort to alter the flavor or make it very addictive, thus getting people to keep coming back to the dealer because they are "hooked". This is not to say that all dealers do this, but a FEW do. So be careful.