Not especially. January 31 is less common than most dates in these historical U.S. records, but it is not among the rarest. It ranks #278 of 366 dates by total births from 1994 through 2014. Rank #1 means most common.
January 31 had 229,472 births across 21 occurrences, or 10,927 births per occurrence. That is 0.268% of the 85,712,738-birth sample, or about 1 in 374 recorded births.
It ranks #12 of the 31 January dates.
Five of its 21 occurrences fell on a Saturday or Sunday. The average across all dates is 6.0. Weekend days have fewer recorded births, so this calendar mix can raise its total. This comes from the calendar in these years, not from the date itself.
Does January 31 stay below nearby same-weekday dates?
Yes, by a small margin. In SSA 2000–2014, the primary comparison gives -3.1%. That means January 31 had 3.1% fewer births than same-weekday dates 7 and 14 days away. It was below its controls in all 15 years.
Every planned alternative comparison has the same direction. With only the 7-day controls, the result is -2.1%. With only the 14-day controls, it is -4.3%. Without holiday exclusions, it is -2.7%.
The difference is small. It describes these records only. It does not show why births differed.
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 January 31 with controls from the same year.
| Comparison | Years | Control observations | Difference |
|---|---|---|---|
| 7 and 14 days, holidays excluded (primary) | 15 | 57 | -3.1% |
| 7 days only | 15 | 30 | -2.1% |
| 14 days only | 15 | 27 | -4.3% |
| 7 and 14 days, no holiday exclusions | 15 | 60 | -2.7% |
How were the comparison dates chosen?
The primary controls are January 17, January 24, February 7, and February 14 in the same year. Each is 7 or 14 days from January 31, so it falls on the same weekday.
In some years, a planned control falls on Martin Luther King Jr. Day. We remove that control under the standard observed federal holiday rules. The year keeps its other controls.
By five-year period, the primary differences are -3.3% for 2000–2004, -2.7% for 2005–2009, and -3.2% 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 January 31 rank change over time?
Somewhat, but less than most dates. We ranked 365 recurring dates by SSA births in each five-year period. January 31 placed #228 in 2000–2004, #178 in 2005–2009, and #241 in 2010–2014. The three ranks span 63 places.
The median span for all 365 recurring dates is 159 places. This span is below that median.
Each period has one weekend occurrence, so the weekend mix does not explain the movement.
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 January guide for the full month ranking. See also the January 2 and January 8 studies.
The methodology explains ranks and shares. Use the birthday checker to compare another date.