Pregnancy. Marijuana. Bad.
Ok, here's some details. Getting pregnant is going to be harder in the first place. Usually, male partners abuse the same substances that their female counterparts do. In men, marijuana decreases testosterone, sperm count, and potency. Needless to say, not a winning combination for conception. Women who smoke marijuana have increased risk of abnormalities in ovulation and therefore more infertility. This risk is enhanced when the use is within one year before attempting conception. This risk did not change with frequency or length of use. So, that means that even occasional use of small amounts of marijuana can decrease women's fertility.
But many women overcome those hurdles. Then, they go on to have higher rates of spontaneous abortion, pregnancy complications, and problems with labor and delivery. Birth defects are associated with marijuana use. Fathers' use lead to higher rates of certain heart abnormalities. Mothers' use is associated with mutant lymphocytes--deranged immune cell factors--which could later lead to higher rates of childhood cancers.
But wait--there's more. Their newborns can experience mild withdrawal and some nervous system effects. And the toxic effects of pre-natal marijuana exposure is the gift that keeps on giving. Effects are often not readily apparent in newborns. Still, psychologists who have studied these babies have found neurological delays. Likewise, few negative effects are apparent between the ages of 1 and 3. But a woman shouldn't be fooled if she has heard of a study where the very young kids show no ill effects. Pre-natal marijuana exposure effects what are called "higher cognitive functions" which don't express themselves until later. At four years old, decreases in verbal ability and memory can be found in those kid exposed to pre-natal marijuana use. At ages 5 and 6, more problems with attention span are found. Needless to say, none of this sets the kid up for a great experience in grade school. At ages 9-12, the same problems persist and compound. Pre-natally exposed to marijuana kids have lower impulse control, don't do as well on certain types of analysis and reasoning tasks, and have lower reading and language skills.
So best advice--quit. Ideally, a woman and her partner should quit long before they try for conception. Should a woman find herself pregnant while still using, she should seek out a treatment program immediately. Quitting any substance abused is difficult, but pregnancy is the strongest motivator for women. There are treatment programs geared towards women, and some even geared towards pregnant women.