Cannabis is the most commonly reported illicit drug of choice among young people and the most commonly used illicit drug by pregnant women.  Studies, which have investigated the effects of cannabis upon unborn children however, are not common.
Since experimental administration of drugs to pregnant women is unethical, studies that do exist have been limited to clinical observations, animal testing or epidemiological investigations.  Although clinical reports alert health care professionals to possible agents causing abnormal development, they are often difficult to evaluate. 

Cannabis and pregnancy

Intelligence

A longitudinal study conducted by Fried (1995) measured the cognitive ability of children born to mothers who smoked cannabis during pregnancy, comparing results with a control group of mothers who did not. Measures of cognitive ability included: language development; reading ability; visual and perceptual tests.  The participants were split into those born to mothers who smoked heavily, moderately and not at all during pregnancy. Findings suggested that there were no differences in cognitive ability.  Initially some small deficits were noticed among children born to cannabis smoking mothers, but by the time the child was 5 years old the deficits could no longer be detected.

A further study by Fried et al (1998) found that children of mothers who smoked cannabis during pregnancy who were between 9 and 12 years old had reduced ability in terms of 'executive functioning' - involving concepts such as planning and anticipation.

Birth weight

A large study focussing on children’s birth weight found that cannabis users did produce babies of lower weight and with a greater chance of malformations (Braunstein et al: 1983).  Studies by Fried (1982) and Hingson et al (1982) however showed that women who smoked cannabis regularly whilst pregnant tended to have babies of a lower weight than those who did not.

Further studies have substantiated such findings, one such report found that offspring from mothers who smoked cannabis whilst pregnant weighed on average 3.4 ounces less than newborn babies whose mothers had not (Zuckerman et al: 1989). The degree to which such findings are actually due to cannabis use during pregnancy, compared to the effects caused by general smoking has not been fully investigated.

Birth defects

The study by Zuckerman et al (1989) also reported that there was also no difference in the gestation period or in the amount of congenital abnormalities reported.  The World Health Organisation has reported that there is reliable scientific evidence to suggest that cannabis causes any chromosomal or genetic damage. Ammenheuser et al (1998) found that mothers who smoked cannabis during pregnancy produced babies with higher mutation rates than those of non-smokers. This is a very similar finding to their 1994 study on mutations in tobacco-smoking mothers. 

One study focused upon on both smokers (classed as light (< 10 joints/week); moderate (11<week>20); and heavy (> 21 joints/week) smokers) and users of the more traditional preparation of cannabis in tea form.  Findings reported that there were no differences in neurobehavioural assessments made between babies born to cannabis ingesting mothers and non-cannabis ingesting mothers (Dreher et al: 1994).

It is therefore difficult to distinguish between the effects of cannabis from the effects of general smoking.

Gestation and labour

A review of existing literature relating to cannabis and pregnancy conducted by Cornelious et al (1995) found that gestation period was shorter in cannabis-smoking mothers, but only in those in their adolescence. Greenland et al (1982) found a greater difference in the duration of labour in mothers who used cannabis than those who did not. Another study however, found no difference between heavy, medium and non-users of cannabis in regard to several neonatal outcomes (Fried et al 1983). Such findings however have to be treated with caution due to small sample sizes.

Tobacco and pregnancy

Evidence on the damage caused by smoking tobacco during is well established.  The following negative effects are widely reported and form the basis of world wide smoking cessation campaigns, which target pregnant women.  The following facts have been taken from the Department of Health’s 2006 Pregnancy Book:

Smoking while pregnant has been linked to:
Premature birth;
Miscarriage;
Low birth weight;
Still birth;
Cot death;
Bleeding and nausea for the mother.

In the first six months of life, babies are also at greater risk of developing asthma, chest infections, and other breathing problems such as wheezing.

Conclusions

It is difficult to draw conclusions on the effects cannabis use on unborn children due to a lack of research in this area however there is little evidence supporting any association between cannabis use during pregnancy and any abnormalities in children. How much of the negative effects of cannabis smoking during pregnancy, which has been reported, is actually due to cannabis compared to the effects caused by general smoking is unknown.  It therefore is hard to distinguish the effects of cannabis from the effects of general smoking, which has been proven to have a range of negative effects on mother and child; pre and post natal.

NB on the date of submission of this answer (6/7/6/) news reports from Australia suggested new research had indicated a link between cannabis use during pregnancy and developmental deficits. A request has been made with the study authors for a copy of the research and this will be forwarded once received. An example news story is reproduced below for information purposes. However, this should not be assumed to be an accurate representation of the research.

ABC news 6/7/6
http://www.abc.net.au/news/newsitems/200607/s1679893.htm

Cannabis use risks unborn babies' health: study

A new study has found that cannabis use by pregnant women has a significant negative effect on their babies' health.
The study was carried out by the National Drug and Alcohol Research Centre.
In what it describes as the largest study of its kind, the research centre analysed more than 400,000 live births over a five-year period, studying the effects of cannabis, opioids and stimulants, on the developing foetus.
Dr Lucy Burns says it showed that cannabis smoking had a major effect.
"Cannabis use seemed to have quite a significant impact on the baby in terms of the baby's weight being lower and born more often prematurely," she said.
Dr Burns says it places those babies at a disadvantage.
"The research suggests that if babies are born early and underweight it is an indicator for long-term negative outcomes, so they are significant indicators of someone who could well have problems later in life," she said.
She says they were also more likely to have to be admitted to neonatal intensive care units.
Dr Burns says while any drug use during pregnancy can be harmful to the unborn child, the effect of cannabis smoking has previously been down-played.
"I guess it's a little surprising in that we found cannabis use seemed to have quite a significant impact on the baby," she said.
Dr Burns says those women who took illicit drugs during pregnancy were also heavy smokers.

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Ammenheuser MM, Berenson NJ, Stiglich EB, Whorton Jr EB, Ward Jr JB. (1994) Elevated frequencies of hprt mutant Iymphocytes in cigarette-smoking mothers and their newborns. Mutation Research 304:285-294.

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