Reported sex ratios at birth for some human populations may be influenced not only by cultural preferences and social practices that favor the birth or survival of one sex over the other but also by incomplete or inaccurate reporting or recording of the births or the survival of infants.[citation needed] Even what constitutes a live birth or infant death may vary from one population to another. For example, for most of the 20th century in Russia (and the Soviet Union), extremely premature newborns (less than 28 weeks gestational age, or less than 1000 grams in weight, or less than 35 centimeters in length) were not counted as a live birth until they had survived 7 days; and if that infant died in those first 168 hours it would not be counted as an infant death. This led to serious underreporting of the infant mortality rate (by 22% to 25%) relative to standards recommended by the World Health Organization.[51]