Thursday January 29 2009
Too early to label PFCs as a cause of infertility
New research suggests that “chemicals found in food packaging, pesticides and household items may be linked to lower fertility among women”, The Times reported. The newspaper said a study of 1,240 women found that those with higher levels of perfluorinated chemicals (PFCs) in their blood, took longer to become pregnant than those with lower levels.
This study looked at levels of two types of PFCs in the blood of pregnant women and asked them how long it took for them to conceive. Although the study found that women with higher levels took longer to conceive, this association does not prove causation. The chemicals in the women’s blood were only measured on one occasion, once they were pregnant. It’s not possible to conclude that the chemicals caused the longer time to pregnancy. Also, these women were all pregnant and therefore cannot be classed as infertile.
There are a wide variety of reasons why women may experience difficulty in becoming pregnant and without further research into PFCs and their possible effects upon the body, it is too early to label PFCs as a cause of infertility.
Where did the story come from?
Chunyuan Fei and colleagues from the University of California, the International Epidemiology Institute, the Vanderbilt University Medical Center, and the University of Aarhus carried out this research. It was funded by the International Epidemiology Institute and the 3M Company. The study was published in the (peer-reviewed) medical journal: Human Reproduction.
What kind of scientific study was this?
The researchers say that perfluorinated chemicals (PFCs) are found in many consumer products and manufacturing processes. They remain in the natural environment for a long time, and have been found in humans and animals around the world. They were considered harmless when they were originally introduced in the 1950s, but animal studies have since found them to have a toxic effect on the liver, immune system and reproductive organs.
This cross-sectional analysis aimed to investigate what effects PFCs may have on fertility. The researchers used data from women enrolled in a larger study, the Danish National Birth Cohort study. They wanted to see whether maternal levels of the PFCs perflurooctanoate (PFOA) and perfluorooctane sulfonate (PFOS), measured in early pregnancy were linked to how long it took to conceive.
The Danish National Birth Cohort is a nationwide study following almost 100,000 mothers and children. In the current study, women who were six to 12 weeks pregnant were identified through their GPs. Pregnancy blood samples were taken from 43,045 women at their first antenatal visit (four to 14 weeks). These were analysed for their concentration of PFOA and PFOS.
The women were also given telephone interviews twice during pregnancy and twice after giving birth. They were asked about their time to pregnancy (TTP) from the time that they first started trying for a baby, to conceiving. Their answers were classified as immediate (within one month), one to two months, three to five months, six to 12 months, greater than 12 months, or if they required infertility treatment to get pregnant.
They were also asked about several factors that can affect TTP including maternal age, BMI, previous children, social status and education, alcohol consumption, father’s age and occupation, menstrual history, and history of miscarriage.
The researchers randomly selected 1,400 women who had provided all the necessary data and who had given birth to a healthy, single baby. After excluding women with unknown conception time/time to pregnancy (TTP) and those with unplanned pregnancies, they were left with a final sample of 1,240 women for analysis.
What were the results of the study?
The average age of women in the sample was 30.6 years and 45% were having their first baby. Half of the women became pregnant within two months of trying to conceive; only 30% took over six months, half of whom (about 15%) took longer than 12 months.
The average concentration of PFOA in the blood was 5.3ng/ml, and PFOS had an average level of 33.7ng/ml. The researchers found associations between the chemical levels and certain factors. These included correlations between decreasing levels of the chemicals and increasing age, increasing number of children and lower BMI.
Women who took longer than six months to get pregnant had significantly higher levels of PFOS and PFOA than women who got pregnant within six months. Women who took longer than six months to conceive were more likely to be older, middle class, and to have a history of miscarriage or irregular menstruation.
When grouped by their PFC levels, there were more women who took longer than 12 months to conceive in the upper concentration levels of PFOS than in the lower concentrations. From this it was estimated that, compared to the lowest blood levels of PFCs, the odds of ‘infertility’ significantly increased with each increasing exposure category of PFCs, and that women with longer TTP had higher exposure to PFCs.
What interpretations did the researchers draw from these results?
The researchers conclude that PFOA and PFOS exposure at normal blood levels observed in the general population may reduce the ability to become pregnant.
What does the NHS Knowledge Service make of this study?
This study is the first to assess the association between blood levels of PFOA and PFOS and time to conception.
- Although this study is strengthened by the fact that it took a large sample of women from a nationwide study, it is weakened by its cross sectional analysis on the data (i.e. the blood samples were taken once and the women were questioned on how long it took to conceive). As such, it cannot prove that one of these factors caused the other. For example, difficulties in conceiving may have been caused by certain medical, personal or psychological factors, and these may in turn also have caused the women to have the higher PFC levels, rather than the higher PFC levels causing the lowered fertility. The research did find that women who took longer than six months to conceive were also more likely to be older, to be middle class, and to have history of miscarriage or irregular menstruation.
- Additionally, not all possible maternal or paternal causes of reduced fertility were assessed or taken into account in the analyses. For example, there was no information on frequency of intercourse or male sperm count, both of which contribute to fertility and TTP.
- Blood levels of the PFCs were only taken once in early pregnancy. It is not possible to say from this study whether blood levels remain stable or fluctuate over time (i.e. a woman with high PFC levels during pregnancy may have had low PFCs levels when she was trying to conceive).
- Time to conceive was self-reported by the women and so its accuracy is not known.
- As PFCs are present in so many consumer products, it is not possible to attribute PFC levels to any one particular exposure, such as certain food packaging or household goods. Therefore, even if higher PFC exposure was associated with lower fertility it would be very difficult to avoid these chemicals. Additionally this study was conducted in Denmark, where environmental levels may not be the same as elsewhere.
- These women were all pregnant and therefore cannot be classed as infertile, so the link between the chemicals and ‘infertility’ or even ‘sub-fertility’, are weak. Information on PFC levels in women who were never able to conceive a child would be valuable.
There are a wide variety of reasons why women may have trouble getting pregnant. Without further research into PFCs and their possible effects upon the body, it is too early to label PFCs as another cause of infertility.