CDC Births Data

This data is drawn from the USA Centers for Disease Control and Prevention, and was compiled via Google's BigQuery Web UI using the following query:

SELECT
  year, month, day,
  IF (is_male, 'M', 'F') AS gender,
  SUM(record_weight) as births
FROM
  [publicdata:samples.natality]
GROUP BY
  year, month, day, gender
ORDER BY
  year, month, day, gender

It is aggregated so as to comply with their terms of use. Data was accessed June 9th, 2015.

Note that this data has been fairly extensively analyzed by Andrew Gelman and his group; see e.g. this post.