The article by Rahman et al., determined the effect of arsenic concentration in contaminated tube well water on 5 outcomes (of infant death and fetal loss) in Bangladesh. This was a cohort study.

Any scientific study can suffer from bias, whether they are in planning stage, data collection stage, data analysis stage or reporting (or publication) stage. In general, there are two major classes of bias: random and systematic. If we assume that no bias was introduced during the publication/reporting stage and that the very large sample size (29,134 pregnancies) will remove any potential bias in sample selection and also that no bias was introduced during analysis stage; we are left with bias during the planning and data collection stages. There are a number of factors that, in my opinion have been left out, could have strongly affected the results. These include, any past history of fetal loss and infant death for a given subject, nutritional practices (availability of adequate nutritional diet), availability of healthcare at appropriate time, changes in arsenic concentration over the duration of study (pregnancy to first 12 months of child birth), availability of water treatment methods (i.e., was the pregnant woman given treated water or regular tube-well water). There could be a number of other factors that could have made the study more robust, by improving the study design.

Hope this helps.

In this study by Rahman et al., effect of arsenic exposure through consumption of contaminated tube-well water on infant death and fetal loss was evaluated. The outcome (or disease) was studied in terms of 

1. Early fetal loss: defined as the loss of fetus within first 28 weeks of pregnancy. This excludes any fetal loss due to induced abortion or vacuum aspiration after missed menstrual period (within first 10 weeks of pregnancy).
2. Late fetal loss or stillbirth: has been defined, in this study, as the birth of a dead fetus after 28 weeks of pregnancy.
3. Neonatal death: was defined as death of an infant within 28 days of birth.
4. Postnatal death: was defined, by the authors, as the death of an infant after 28 days of birth, but within first 12 months of age.
5. Infant death: was defined as the death of an infant within first 12 months of birth.

The study used a large sample set of 29,134 pregnancies to determine the outcomes as a function of arsenic concentration in the tube wells that were used by sample subjects.

Hope this helps.