Not especially. May 18 is less common than most dates in these historical U.S. records, but it is not among the rarest. It ranks #276 of 366 dates by total births from 1994 through 2014. Rank #1 means most common.
May 18 had 229,518 births across 21 occurrences, or 10,929 births per occurrence. That is 0.268% of the 85,712,738-birth sample, or about 1 in 373 recorded births.
Within May, May 18 ranks #22 of 31 dates.
Seven 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 lower its total. This comes from the calendar in these years, not from the date itself.
Does May 18 differ from nearby same-weekday dates?
May 18 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 -0.2% to -1.0%.
Some comparisons differ by less than 1%, so we treat the result as weak.
In the primary comparison, May 18 was above its controls in five years and below them in ten.
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 May 18 with controls from the same year.
| Comparison | Years | Control observations | Difference |
|---|---|---|---|
| 7 and 14 days, holidays excluded (primary) | 15 | 59 | -0.8% |
| 7 days only | 15 | 29 | -1.0% |
| 14 days only | 15 | 30 | -0.6% |
| 7 and 14 days, no holiday exclusions | 15 | 60 | -0.2% |
How were the comparison dates chosen?
The primary controls are May 4, May 11, May 25, and June 1 in the same year. Each is 7 or 14 days from May 18, so it falls on the same weekday.
In some years, a planned control falls on Memorial 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 0.0% for 2000–2004, -0.8% for 2005–2009, and -1.7% 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 May 18 rank change over time?
Yes. We ranked 365 recurring dates by SSA births in each five-year period. May 18 placed #276 in 2000–2004, #167 in 2005–2009, and #294 in 2010–2014. The three ranks span 127 places.
The median span for all 365 recurring dates is 159 places. This span is below that median.
Each period has only five occurrences of May 18. 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 May guide for the full month ranking. Compare the nearby May 5 study.
The methodology explains ranks and shares. Use the birthday checker to compare another date.