Researchers have found a link between the outcome of fetal health when exposed to carcinogens and when the fetus was exposed during gestation. The study was completed by researchers at the Linus Pauling Institute (Oregon State University). The journal Cancer Letters published the study.
Mice were used for research purposes. The mice were exposed to a chemical carcinogen found in pollutants and combustibles. Four doses of the carcinogen were given to mice. At middle age, the risk of ovarian cancer in female mice tripled. Male mice presented with smaller than normal testes in some cases, a reaction never documented before in clinical research and all mice were at increased risk of lung cancer with 80-percent of mice contracting the cancer.
In a comparable study, the same carcinogen was given to mice in a single dose. In this study, mice were more apt to develop blood cancer, not the forms of cancer present in the first study. If the exposure to the carcinogen was spread out – the risk of blood cancer and other forms of cancer was reduced significantly.
According to David Williams, a professor at Oregon State University, “There's still a lot of uncertainty about how the fetus responds to carcinogens and at what points in time it is most vulnerable. We know it's far more sensitive than adults for several reasons, including faster cell division and the lack of protective detoxifying enzymes. But it's interesting that the timing of fetal exposure makes such a difference in which organs are targeted. These results were somewhat surprising.”
It is important to note that the mice in the study were exposed to higher amounts of a commonly found air pollutant that humans would ever encounter in real life. The mothers were exposed to the carcinogen, which eventually made its way to the fetus. However, only 10% of the concentration found in mother mice tissue was later found in the fetus. 
When the results were compared to current research on humans, scientists noted that common carcinogens like cigarette smoke also increase the risk of certain forms of cancer in offspring. Doctors associated with the study suggest pregnant women eat a healthy diet rich in fruits and vegetables to reduce the risk of cancer.
Source: David Williams, et al. Oregon State University. 20 December 2011.