The release of the corrected birthweight files describes the problem with the original data processing:

"Raw data for pounds were entered into a two digit field. During data processing, only one digit was read, so that birth weight values of 10-15 pounds were changed to 1 pound, misclassifying high birth weights to very low birth weights. . .In addition, if an interviewer entered 08 for pounds, it was changed to "0", which was then categorized as "not ascertained". The misclassifications were compounded when the incorrect pound values were translated into grams. The ounce data were processed correctly. . . There were additional problems associated with the data entry and processing for cases where birth weight was originally reported in grams. However, it is important to note that relatively few respondents report birth weight in grams (about 100 cases or less than 1% in 2004). . . From 2004-2010, initial data entry allowed birth weight values of 500-6900 grams. In the editing process the range was set to 500-5485 grams, and responses greater than 5485 grams were changed to "Don't know". Responses of less than 500 grams were not allowed by the instrument and are not present in the raw data files."

The re-released files affect data extracts made before September 2015 and only alter the 2004-2010 samples. Users interested in comparing the original birthweight files should contact IPUMS NHIS user support to obtain the previous, incorrect values for these years. Original NCHS documentation on the corrected birth weight files is available here.