Although the pregnancy rate was much lower in heavy smokers, for those that did become pregnant the multiple pregnancy rate was much higher, with 60% of the heavy smokers expecting twins against 31% of the non-heavy smokers.
"This is counter-intuitive," said Dr Soares, "and although we controlled for all known interfering variables, it is possible this is not a real association. What would confirm it as a real association is to find it still applies in a large number of cases. However, if it is confirmed as a real association it suggests that, paradoxically, tobacco constituents affect the uterus in different ways in different women, impairing implantation in some and having the opposite effect in others.
"It is already known that there is paradoxical dose-dependent effect of nicotine on ovarian tissue. Maybe the same effect is happening in uterine tissue. The effect of a certain molecule depends, not only on its concentration, but also on the concentration of its receptors and all the other molecules present in the cell. It could be that heavy smoking disrupts the stability of cells in the lining of the uterus differently in some women to others or triggers a response in the embryo itself, resulting in a reduced general pregnancy rate overall, but an increased chance of multiple pregnancy in those who do become pregnant."
Dr Soares said, that as well as confirming these findings in higher numbers of heavy smokers who use donated oocytes, it would be worthwhile studying differences in gene expression in the lining of the uterus of heavy smokers who fail to get pregnant and those whose have twins.
But, for now, the clinical implications of the findings were clear. Tobacco makes the uterus less likely to accept an embryo.
"In spite of all the noise generated about the effects of smoking in a series of health areas, its possible effect on uterin
Source:European Society for Human Reproduction and Embryology