Here you are:
1) the WHO admits that measures in countries it collects data from are often not taken in a consistent manner. "Monitoring progress is challenging in light of weak health information systems in some countries. Problems such as underreporting and differences between official data and estimates made by international agencies are some of the contributing factors."
2) We have "... anecdotal evidence suggests that countries do not use consistent practices in measuring these data [2] [3]"
I'll have to do more digging. These are hard to find references. There may be a hard copy of [2] at my university's law library.
[0] http://www.euro.who.int/en/health-topics/Life-stages/materna...
[1] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4193257/#__sec2...
[2] https://www.ncbi.nlm.nih.gov/pubmed/12343088
[3] Hartford RA. New Perspectives on Infant Mortality in the United States: Findings from International Research Studies. Presented at the Association for Social Sciences in Health Meeting at the National Institutes of Health; Bethesda, MD.. Feb. 4, 1992 <-- What's interesting here is that the authors of [1] left a link to scholar.google.com, with no specific reference. This may take a while to track down.