Cannabis and Pregnancy


THC is a substance which passes from the mother’s blood to that of the foetus. This means that THC can cause direct damage to the foetus during pregnancy. (THC is also passed on to the infant via breast-milk.) Animal experiments have shown a number of very serious effects on the gestation of, and on the young born to, females which have been given cannabis or THC during gestation. These findings have naturally given rise to questions concerning the risks to which the human foetus is exposed if the mother smokes hashish or marijuana during pregnancy ().

Researchers are here confronted with the usual problems of finding a reliable design for their studies, including comparable control groups. Further, research of this type is faced with particular difficulties as regards finding means of excluding other factors that might explain damage that has been observed, such as other drugs (including alcohol and tobacco), inadequate nutrition and infections during pregnancy. Moreover, it is also difficult to find suitable methods of measurement with sufficient sensitivity to detect even damage of a subtle nature.

The conceivable – and suspected – harmful effects of cannabis can be divided into the following categories:

  • Effects on birth weight, birth height, etc. (similar to the effects of tobacco smoking during pregnancy)
  • Increased risk of malformation
  • Effects on the central nervous system of the foetus
  • Other effects on the foetus

Effects on Birth Weight etc.

Until the mid-1980s, there were doubts as to the effects of cannabis on factors such as birth weight. Early studies produced varying results and were not always conducted with sufficient thoroughness. A series of later studies have shown that cannabis smoking during pregnancy is statistically associated with a lower average birth weight () and a shorter average body height.

However, findings which in certain respects call into question the effect of both cocaine and marijuana on birth weight have been reported in a multi-centre study (). This study does not find any significant association between marijuana smoking during pregnancy and a lower birth weight of the child. If one looks solely at the association between mothers found to have marijuana in their blood during pregnancy and the birth weight of their children, though, there turns out to be a clear tendency which points in the same direction as the studies mentioned above. The overall picture is that – when account is taken of the quality of different studies, the methods they use to establish current abuse and the selection of pregnant women for the respective studies – cannabis use, at least habitual cannabis use, during pregnancy represents a risk that the pregnancy and the foetus will be affected, with reduced birth weight and birth height as a consequence.

The importance of these effects for the children’s further development, however, is not known. The central result achieved is that it has been demonstrated that THC, by affecting the mother’s hormonal system (and reducing the duration of pregnancy) or by its direct toxic action, produces such a clear and measurable effect on foetal development.

Increased Risk of Malformation

The term “malformation” here refers to abnormal anatomical developments of parts of the body and/or internal organs that are visible to the naked eye or can be detected by means of traditional examination methods used in radiology or laboratory diagnosis (to determine functional disturbance).

The increased frequency of malformation shown in animal experiments has not been as striking as the inhibition of growth mentioned above; and moreover, it has been necessary to use very high doses to provoke that increased rate of malformation.

The research findings in this area are somewhat contradictory. While there are a few studies () which suggest a higher risk of malformation, most studies – and indeed the best-designed and best-executed ones – have produced findings which contradict the suspicion that the smoking of cannabis preparations increases the risk for malformation of parts of the body or of internal organs ().

That being said, since there do exist studies which have arrived at a different conclusion, since the “exonerative” studies have certain methodological shortcomings, and since most of the “exonerative” studies relate to marijuana and were carried out during the first half of the 1980s or earlier – when marijuana with low THC concentrations was still in widespread use –, it would be unwise to exclude cannabis as a cause of malformation until larger and better-controlled studies have been carried out ().

Malformation as a Result of Chromosomal Damage Caused by Cannabis

Malformation can also be caused in other ways than through a direct toxic effect on the foetus. By damaging the genetic material of either parent, poisons capable of affecting the genes can cause malformation genetically.

It has not been possible to prove that THC can produce such effects, and this risk has been dismissed in several reviews of research ().

Damage to the Central Nervous System of the Foetus

It seems a reasonable suspicion that cognitive damage could arise at the foetal stage, bearing in mind the acute effects of cannabis on cognitive functions as well as the damage to cognitive functions caused in adults and young people by long-term exposure to cannabis (). It is indeed within the area of effects on the central nervous system that the most disturbing scientific findings have been made.

A central position in this field of research is occupied by the Ottawa Prenatal Prospective Study (OPPS), carried out by a research team led by Peter Fried. This study of the children of mothers who smoked marijuana while pregnant is unique, not least because of its long-term nature. The mothers were recruited to the research programme during the period 1978–1983. The children were examined from a neurological point of view immediately after birth and on several occasions during their first year; thereafter, they have been tested with regard to cognitive and psychomotor functions once a year up to the age of 16.

The researchers initially found signs of deficiencies in the children’s neurological development, or signs of withdrawal effects. These symptoms disappeared during the first year, and when the children were examined with regard to motor development, perception and motor functions at the ages of one, two and three years, no deficiencies were found which could be related to their exposure to cannabis during the foetal stage. However, when the children were examined at the age of four, deficiencies were found in their memory and verbal ability. These deficiencies were no longer detectable at the ages of five and six – although the six-year-olds were found to have impaired ability to maintain attention. In the examinations at ages six to nine, several manifestations of deficiencies in cognitive functions were identified. The parents of the children who had been exposed to marijuana were also more likely to report behavioural disturbance in their children.

When the children were examined at ages nine to twelve, moderate functional impairment of a specific type was discovered: the children had reduced ability as regards memory in connection with visual stimuli, analytical ability and integrative ability. Moreover, attention disturbances were found. The same pattern recurred at ages 13–16 (). It could be mentioned that the disturbances exhibited by children whose mothers had smoked cannabis while pregnant were different from the disturbances manifested by children whose mothers had smoked regular cigarettes only; the latter children were affected as regards overall intelligence and certain functions related to hearing.

To sum up, the research team found that – in addition to slight, transitory neurological deficiencies at birth – the children showed a slight disturbance to cognitive functions which was not detectable until they reached the age of four, and which subsequently disappeared. Other kinds of disturbance to cognitive functions, together with behavioural problems, appeared during the children’s first school years and have proved possible to follow until the age of 13–16.

Fried () presents a hypothesis – which tallies well with the kinds of cognitive damage sustained by adults following long-term cannabis smoking – as to why the deficiencies are not detectable until a few years after birth: the damage caused at the foetal stage is assumed not to become important until the child needs to function at a higher level with regard to “executive” functions (integrative cognitive functions of importance for processes such as problem-solving and planning). Indeed, cognitive disturbances of these kinds are among those found in adults ().

Most studies of the effects resulting from cannabis use during pregnancy do not follow the child beyond the age of one year. This is why there is only a very limited amount of scientific data either supporting the findings reported by Fried et al. or calling them into question. Day et al. () found an association between marijuana smoking during pregnancy and lower intelligence-test scores at the age of three (but not before that age); this finding supports those of Fried. An association between marijuana smoking during pregnancy and sleeping problems in three-year-olds detected by Dahl () also points in the same direction.

Peter Fried () warns us against underestimating the risks to the foetus from cannabis exposure during pregnancy. He emphasises that his study looked at the effects arising from marijuana smoking in the 1970s, reminding us that the marijuana in use today has a considerably higher THC content.

Recently, however, another long-term study has become available to us, even though the children included in it are five to six years younger. Goldschmidt et al. () studied a group of just over 600 pregnant women, of whom slightly less than half smoked marijuana in different amounts while pregnant. Careful assessments of aspects such as the situation in the women’s homes were carried out during the pregnancy and after they had given birth, in order to isolate the effects, if any, of cannabis. During the first years, the main emphasis was placed on reports from parents and, later, teachers. At the age of six, an association was found between exposure to marijuana and teacher reports of delinquent-behaviour problems.

When the children were ten years old, a more extensive assessment was carried out, including interviews with parents and teachers as well as standardised questionnaires. A clear association was found between exposure and “delinquency”. Further, it was found that these behavioural disturbances were mediated by pronounced hyperactivity, impulsiveness and deficiencies in attention. These characteristics proved to be associated with the degree of exposure to marijuana.

A comparison of these two long-term studies – probably the only ones of their kind in the world – yields several similar tendencies. One of them is that test-score differences and behavioural disturbances, respectively, appear at a relatively late stage, as a manifestation of the fact that the damaged functions do not develop and start to be used until then. Further similarities include reports by parents and teachers, respectively, of disturbed/ restless behaviour, which appear at more or less the same age. While Fried has focused more on measuring cognitive functions, Goldschmidt records behaviour.

Other Effects on the Foetus

Several researchers have recorded a few cases of rare cancers in children of mothers who smoked marijuana while pregnant. While these findings do not prove anything, they do signal a need to be more observant and represent a call for more research (Cannabis: A Health Perspective and Research Agenda, 1997).


THC () is a substance which passes from the mother’s blood system to that of the foetus, which means that THC can produce direct harmful effects on the foetus during pregnancy.

There is a risk that habitual cannabis abuse during pregnancy may affect the foetus, with resulting lower birth weight and shorter birth height.

As regards the risk of foetal damage in the proper sense, research findings are contradictory, but most studies show that cannabis smoking does not increase the risk. However, researchers warn us against excluding cannabis as a cause of malformations before larger and methodologically better-conceived studies have been conducted. That being said, it does seem that we can exclude the risk of malformations being caused by chromosomal damage produced by cannabis.

The most worrying scientific findings concern – not surprisingly – damage to the child’s central nervous system. A very long-term Canadian study found that children of cannabis-smoking mothers had sustained damage to cognitive functions which did not become noticeable until the children reached the age of four. The reason for the late appearance of this damage is assumed to be that the functions involved are “executive” cognitive functions which are not taken into use until the child is four to six years old.


Selections from the book: “Adverse Health Consequences of Cannabis Use. A Survey of Scientific Studies Published up to and including the Autumn of 2003