Not especially. April 21 is less common than most dates in these historical U.S. records, but it is not among the rarest. It ranks #306 of 366 dates by total births from 1994 through 2014. Rank #1 means most common.
April 21 had 227,759 births across 21 occurrences, or 10,846 births per occurrence. That is 0.266% of the 85,712,738-birth sample, or about 1 in 376 recorded births.
It ranks #21 of the 30 April dates.
Does April 21 differ from nearby same-weekday dates?
April 21 has slightly fewer births than its controls in every planned comparison. In SSA 2000–2014, the primary comparison gives -0.8%. The planned comparisons range from -0.3% to -1.3%.
Some comparisons differ by less than 1%, so we treat the result as weak.
In the primary comparison, April 21 was above its controls in seven years and below them in eight.
Baseline ranks use historical U.S. CDC/NCHS and SSA records, 1994–2014. The percentage differences use SSA 2000–2014 alone. Each table row compares April 21 with controls from the same year.
| Comparison | Years | Control observations | Difference |
|---|---|---|---|
| 7 and 14 days, holidays excluded (primary) | 15 | 60 | -0.8% |
| 7 days only | 15 | 30 | -0.3% |
| 14 days only | 15 | 30 | -1.3% |
| 7 and 14 days, no holiday exclusions | 15 | 60 | -0.8% |
How were the comparison dates chosen?
The primary controls are April 7, April 14, April 28, and May 5 in the same year. Each is 7 or 14 days from April 21, so it falls on the same weekday.
None of these controls fell on an excluded holiday. We exclude holidays under the standard observed federal holiday rules. We also exclude Christmas Eve, December 26, and the Friday after Thanksgiving.
By five-year period, the primary differences are -2.2% for 2000–2004, -0.5% for 2005–2009, and +0.3% for 2010–2014.
To combine the years, we divide the summed target births by the summed annual control means. Then we subtract one and multiply by 100. The result is not an average of annual percentages.
Does the April 21 rank change over time?
Yes. We ranked 365 recurring dates by SSA births in each five-year period. April 21 placed #350 in 2000–2004, #158 in 2005–2009, and #324 in 2010–2014. The three ranks span 192 places.
The median span for all 365 recurring dates is 159 places. This span is above that median.
Each period has only five occurrences of April 21. It fell on a weekend twice in 2000–2004, twice in 2010–2014, and once in 2005–2009. The periods with more weekend occurrences have less common ranks.
These ranks use raw birth counts. The ranking-stability study also compares annual shares and weekday-relative scores for every date.
Sources and limits
Baseline totals and ranks use FiveThirtyEight’s pinned U.S. birth files. We use CDC/NCHS for 1994–1999 and SSA for 2000–2014, so no year comes from both sources. Annual comparisons use SSA alone.
The source series have different administrative coverage. These records do not describe worldwide births, everyone alive today, or current birth rates.
The daily totals do not record delivery methods or scheduling decisions. Weekday matching does not remove every seasonal difference. Federal holiday rules identify calendar dates, not hospital schedules.
Use the April guide for the full month ranking. See also the April 1 study.
The methodology explains ranks and shares. Use the birthday checker to compare another date.