For obvious reasons, the research available for the direct impact of maternal drug use on pregnancy is patchy. What evidence there is suggests that most illegal drugs are, unsurprisingly, potentially damaging for a developing baby. Studies have shown the use of many illicit drugs to be associated with an increased risk of developmental problems and abnormalities, premature birth and even miscarriage and stillbirth. However, studies also show the importance of high-quality prenatal care to positive outcomes for these higher-risk pregnancies.
What are the risks to my baby if I use drugs?
What little evidence there is relating to marijuana use during pregnancy is conflicting, and most studies face difficulties separating out the effects of marijuana from the effects of other drugs frequently taken in association with it, such as alcohol and tobacco. Some studies have linked use of the drug with slow fetal growth rates and lower birth weights, while other studies - including one published in the British Journal of Obstetrics and Gynecology in 2002  - have found that there is no significant difference between the birthweights and growth rates of babies exposed to maternal cannabis use and those not exposed to the drug. At least one study in the US has linked heavy marijuana use in pregnancy with unusual newborn behaviour (excessive crying and trembling). There is no evidence linking the use of marijuana by a pregnant woman with birth defects and abnormalities.
However, there has been much speculation about the potential impact of marijuana use during pregnancy on a child's subsequent development, particularly in terms of the impact it may have on a child's academic performance, ability to concentrate and susceptibility to depression and other mental illness. So far only very limited evidence has been gathered, and the speculated negative impact of fetal exposure to marijuana on development has yet to be proved or disproved.
What is clear, however, is that tobacco use during pregnancy holds proven health risks for a developing fetus: Mixing marijuana with tobacco for smoking purposes therefore poses a definite health risk to your baby. You should also bear in mind that marijuana is often mixed with other chemicals without the users' knowledge, and these chemicals may carry proven health risks to a fetus.
Unlike the cases of alcohol and tobacco use, there is currently no overwhelming evidence that using marijuana itself during pregnancy will harm your baby. However, given the lack supportive evidence either way, and the fact that the chemical component in marijuana can pass through the placenta to your fetus, it makes sense to avoid the drug for the duration of your pregnancy. If you have smoked marijuana before realising that you were pregnant then don't worry, it is very unlikely that you will have damaged your baby. From here on in you would be best advised to err on the side of caution and cut out use of the drug as far as possible.
Studies have associated maternal cocaine use with a broad range of health problems for the developing baby. Cocaine taken by the mother can cross the placenta to the developing fetus and may affect fetal development, possibly causing brain damage and even death. Cocaine potentially restricts the flow of blood to the baby, and so could limit the nutrients that a baby receives. While that may be a theoretical risk to the baby, it's clear that as an appetite suppressant cocaine can also make poor maternal nutrition far more likely than otherwise, so affecting the healthy development of the fetus.
Maternal cocaine use has been linked to a higher incidence of miscarriage in early pregnancy, to stillbirth in late pregnancy and to premature birth and low birth weight. Babies born to regular cocaine users are twice as likely to be born prematurely. These babies are consequently exposed to the broad health risks associated with low birth weight and premature birth. While use of cocaine will not necessarily result in developmental abnormalities for the fetus, there is evidence suggesting that exposed babies could be at a higher risk of developing certain abnomalities including defects in the brain, face, eyes, heart, urinary tract and limbs.
Some studies have concluded that maternal cocaine use can raise the risk of placental abruption. This is the condition where the placenta prematurely detaches itself from the uterus leading to heavy bleeding, and which can be fatal for both mother and baby. However, conflicting results from numerous studies on this subject suggest that the higher risk shown to for some cocaine users may be associated with a parallel use of other drugs such as tobacco and alcohol. There may also be a connection between the timing of cocaine exposure and placental abruption, with cocaine taken close to the time of delivery more likely to raise the risk of abruption.
Babies of cocaine users may be at a considerably higher risk of Sudden Infant Death Syndrome (SIDS), with an increased risk as high as 15%. However, studies which support this theory haven't discounted the influence of other factors such as parallel tobacco use and low income, which may account for some, if not all of the risk increase.
Heroin taken by a pregnant woman readily crosses the placenta to developing baby and may seriously affect healthy development. Heroin use during pregnancy is associated with an increased risk of various pregnancy complications such as poor fetal growth, low birth weight, prematurity and premature breaking of the waters, and stillbirth. According to a 2005 study , as many as half of the babies born to heroin addicts have low birth weights and are so exposed to the health risks low birth weight entails. The same study also found an association between maternal heroin use and birth defects, which could be related to the general health of heroin users and the effects of chemicals mixed with heroin, as well as to the drug itself.
After birth, babies born to heroin users have been observed to display withdrawal symptoms such as fever, trembling, irritability and excessive crying, diarrhea and vomiting. As with cocaine, heroin use has been linked with an increased risk of SIDS for babies.
If you are a heroin user it's important that you don't try to stop taking the drug yourself during pregnancy: You will need the help of your doctor to come off the drug safely, with he appropriate therapy, without putting your baby at risk.
Amphetamines (such as ecstasy), Ketamines, LSD and PSP
As the wide-scale use of amphetamines as recreational drugs is a relatively recent phenomenon, there have been even fewer studies into the potential impact on a pregnancy. What little evidence there is suggests that use of the drugs is linked to poor fetal growth, premature delivery, low birth weight and placental problems, with the corresponding risks and difficulties these entail.
The impact of Ketamines, LSD and PSP is similarly under-researched, but use of these potent drugs presents a serious health hazard at any time and is certainly best avoided during pregnancy.
Should I stop using recreational drugs if I am pregnant or would like to become pregnant?
In most cases the answer to this question is 'yes'. If you aren't yet pregnant then you would be best advised to use contraception until you have weaned yourself off an addiction to an illicit drug.
If you are already pregnant then in most cases you should do your best to cut out use of the drug. Even where the hard evidence against a particular drug is less strong than that against alcohol and tobacco use during pregnancy, at best you are exposing your baby to an unnecessary, if less reliably quantified, risk. In the case of heroin, however, you shouldn't try to come off the drug without the support of your doctor who can provide methadone therapy.
Should I tell my doctor / midwife about my drug use?
Yes, absolutely. The results of reports into the impact of recreational drug use on pregnancy and fetal development are often limited and conflicting, not least because of the difficulty of obtaining accurate, reliable, unbiased data and of separating out the influence of individual drugs which are often used together. However, what is clear in most studies is that the quality of antenatal care can play a strong role in reducing the likelihood of the many health problems associated with drug use in pregnancy. If you make sure that your doctor is aware of your addiction or habit, s/he will be far better placed to give you the best, most tailored, antenatal care and support.
If you find that your doctor or midwife isn't supportive or helpful then you may want to consider health practitioners.
 Fergusson DM, Horwood LJ, Northstone K. Maternal use of cannabis and pregnancy outcome. British Journal of Obstetrics and Gynaecology 2002
 National Institute on Drug Abuse. Research Report Series–Marijuana Abuse. Updated 10/13/05
 Briggs, G.G., et al. Drugs in Pregnancy and Lactation 7th edition. Philadelphia, PA, Lippincott Williams and Wilkins, 2005.