Longnecker, MP, MA Klebanoff, JW Brock, H Zhou, KA Gray, LL Needham and AJ Wilcox. 2002. Maternal Serum Level of 1,1-Dichloro-2,2-bis(p-chlorophenyl)ethylene and Risk of Cryptorchidism, Hypospadias, and Polythelia among Male Offspring. American Journal of Epidemiology 155:313–22.

Androgenic hormones like testosterone are crucial to the development of the male reproductive tract in all mammals. Laboratory experiments with animals have shown conclusively that compounds interfering with androgen action—anti-androgens—can disrupt the normal development of males. Some of the effects produced include hypospadias, cryptorchidism and polythelia. DDE, a metabolite of DDT, is one such anti-androgen. Each of these birth defects can be produced reliably in substantial percentages of exposed animals via exposure to DDE.

Public health records analyzed by the US Centers for Disease Control and sister agencies around the world have suggested that some male reproductive birth defects, including hypospadias and cryptorchidism have increased in frequency during the latter half of the 20th century, in concert with the widespread use of hormonally-active compounds like DDT. Hence because of the laboratory work on anti-androgens in animals, the rise in use of anti-androgenic compounds and the increases in frequency of male reproductive tract birth defects, scientists have hypothesized that anti-androgenic compounds are contributing to these human health effects.

This new publication by Longnecker et al. attempts to test this hypothesis. It is the second in a series that take advantage of samples of umbilical cord blood stored since the 1960s, combining sophisticated chemical analysis with data on birth outcome. The first of these studies revealed a strong relationship between DDE levels and the likelihood of preterm birth and low birth weight.

Now Longnecker et al. present an analysis suggesting inconclusive links between DDE levels in umbilical cord blood and the likelihood of three male reproductive tract birth defects, hypospadias, cryptorchidism and polythelia. There may be elevations in the risk of these birth defects,... but the statistical analysis cannot rule out the possibility of no association. The suggestion of links to cryptorchidism and polythelia are stronger than those to hypospadias. Possible explanations for this finding are discussed below, after a description of their methods and results.

What did they do?

The blood samples were obtained as part of the Collaborative Perinatal Project, research involving women recruited at 12 different study centers in the US between 1959 and 1966. Blood samples were collected every 8 weeks during pregnancy from the approximately 42,000 women who participated, and then once again 6 weeks post-partum. The children were then assessed for birth defects and other outcomes through the age of 7 years.

The blood samples had been stored for over 30 yrs when Longnecker et al. began a chemical analysis in 1997. The samples had been kept frozen at –20°C since collection; records indicate there was no thaw during that during storage.

Longnecker et al. measured serum levels of p,p-DDT and p,p-DDE in samples taken during the third trimester of pregnancy using electron capture detection after solid-phase extraction, cleanup and dual-gas column chromatography.

They then examined statistical associations between the levels of DDE and the occurrence of the three birth defects using a case-control design with one control group. The analysis used statistical procedures to control for a series of potentially confounding variables, including race, season of birth, mother’s
age, parity, socioeconomic index, body mass index before pregnancy, weight gain during pregnancy, smoking during pregnancy, hypertension, age at menarche, history of infertility, menstrual cycle irregularity, estrogen use during pregnancy,
progesterone use during pregnancy, method of delivery, and
serum sodium level.

What did they find? In the 22,347 boys of the study, the authors found records of 241 cases of cryptorchidism, 214 cases of hypospadias, and 185 cases of polythelia. They then constructed a control group of 599 boys without recorded birth defects, selected otherwise at random from the study. A small number of the case group was subsequently excluded because of details of the chemical analysis. The authors were then left with sample sizes of: 219 cryptorchids, 199 with hypospadias, 167 with polythelia and 552 controls.

The median level of DDE measured in the control group, 34.3 µg/liter (adjusted for proportion recovered) was approximately 5-fold the current level in the United States.

For each of the 3 birth defects, results suggested an increase in risk associated with higher DDE levels. The adjust odds ratio reflected an modestly increased risk for the highest exposure groups, but the 95% confidence intervals were wide and included 1 (that is, they could not exclude the possibility on a statistical basis that the risk was not elevated).

The following table presents this element of Longnecker et al.'s findings. It shows the adjusted odds-ratio for the highest exposure group (> 60 µg/liter DDE) for each birth defect along with 95% confidence intervals.

The strongest association was for polythelia: the median level of DDE in cord blood for boys with polythelia was elevated compared to controls and the adjusted odds ratio for the highest exposure group compared to controls was 1.7.

What does it mean? These results are inconclusive. There were suggestions of modest elevations in adjusted odds ratios, but, as noted above, the confidence intervals were wide and included 1. This means that the authors cannot exclude the possibility that there is no association, in other words a negative result.

As noted above, the DDE levels found in these cord blood samples is significantly higher than current levels. This decline has occurred at the same time as increases in the frequency of hypospadias and cryptorchidism. The combination of these opposite time trends and the lack of unambigous association between DDE levels and hypospadias and cryptorchidism odds-ratios would seem to argue that the hypothesis linking these should be rejected.

There are at least 2 significant challenges to this interpretation, however.

First, the current result may be a "false negative," i.e., that the statistics indicate there is no relationship when in fact there is. Two factors at work here would be consistent with a false negative.

• The sample sizes were small, rendering the statistics unfortunately weak even if elegant.
• DDE is only one of a series of known compounds with anti-androgenic characteristics. Without a more complete picture of the exposure history of the fetus, it is impossible to say that other compounds did not confuse the results. Some of the instances could have been caused by one anti-androgen; the others by DDE. Data looking only at DDE would predict the incidence of a birth defect very imprecisely, and the small sample size would make it very difficult, if not impossible, to detect the effect of DDE by itself.

Second, the fact that hypospadias have been increasing while body burdens have been increasing can also be misleading. Again, there are many more anti-androgens than just DDE. Exposure to others, particularly several phthalates, has is likely to have been increasing (because of increased production volume of phthalates and use in a broader variety of products, especially such consumer products as cosmetics). It would be highly desirable to have information on time trends of exposure to the total pool of anti-androgens. It is possible that total exposure has increased even as the contribution of one anti-androgen, DDE, to that total, has decreased.

As the authors conclude, their results leave unanswered the question as to whether DDE or other anti-androgens contribute to the causation of reproductive birth defects in boys. It remains a plausible hypothesis:

Some definitions

hypospadias: instead of being at the tip of the penis, the urethral opening is somewhere along the shaft or at the base. More...

cryptorchidism: the failure of one or both testes to descend at birth

polythelia: the presence of extra ("supernumerary") nipples.