Our thanks to G for sending us a PDF of a paper published by the Royal College of Obstetricians and Gynaecologists three years ago, Cannabis use during pregnancy in France in 2010. From the ‘Results’ section:

In all, 1.2% of women reported having used cannabis during pregnancy. This percentage was higher among younger women, women living alone, or women who had a low level of education or low income. It was also associated with tobacco use and drinking alcohol. Cannabis users had higher rates of spontaneous preterm births: 6.4 versus 2.8%…

The full ‘Strengths and limitations’ section:

The 2010 survey was the first time that the National Perinatal Survey collected data on cannabis use. The size of the sample, and its representativeness at the national level, constitute the main strength of this study. We have no laboratory tests to validate the women’s self-reports. It is likely that cannabis consumption was under-reported, as its harmfulness is widely suspected by the women. [J4MB emphasis] Such under-reporting would mean that some cannabis users are included in the group considered to be non-users, so that the associations with preterm or small-for-gestational-age births are probably underestimated, compared with the true associations. [J4MB emphasis]