What Happened In the 1920s?

Via Calculated Risk, here's a chart of births per year in the United States:

This is the raw number of births, not births per person or births per woman of child-bearing age or whatnot. Presumably the steep decline in the 1960s and 1970s is a combination of the end of the baby boom and the rise of second-wave feminism. But what really interests me is the earlier decline in the 1920s, which began only a few years after the end of WWII WWI and continued through the Great Depression. What's going there? First-wave feminism? A change in the ethnic mix of immigrants? Influenza?

Update: A reader writes in to point out that the 1920s saw the beginnings of non-Pill birth control advocacy. This gets swept way in popular history because of the more popular Suffrage Movement, a post-war anti-feminist "backlash" (I hate that term), and the relationship between the early contraception advocates and eugenics.