Not especially. March 18 is less common than most dates in these historical U.S. records, but it is not among the rarest. It ranks #242 of 366 dates by total births from 1994 through 2014. Rank #1 means most common.
March 18 had 230,793 births across 21 occurrences, or 10,990 births per occurrence. That is 0.269% of the 85,712,738-birth sample, or about 1 in 371 recorded births.
Within March, March 18 ranks #16 of 31 dates.
Does March 18 differ from nearby same-weekday dates?
March 18 has slightly more births than its controls in every planned comparison. In SSA 2000–2014, the primary comparison gives +1.4%. The planned comparisons range from +0.1% to +2.7%.
The 7-day comparison differs by less than 1%, so we treat the result as weak. The primary and 14-day comparisons include April 1, which ranks #352 of 366 dates in the baseline. March 18 had 6.0% more births than April 1. It had more births than April 1 in all 15 years. Without April 1, the primary controls give -0.1%.
In the primary comparison, March 18 was above its controls in eleven years and below them in four.
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 March 18 with controls from the same year.
| Comparison | Years | Control observations | Difference |
|---|---|---|---|
| 7 and 14 days, holidays excluded (primary) | 15 | 60 | +1.4% |
| 7 days only | 15 | 30 | +0.1% |
| 14 days only | 15 | 30 | +2.7% |
| 7 and 14 days, no holiday exclusions | 15 | 60 | +1.4% |
How were the comparison dates chosen?
The primary controls are March 4, March 11, March 25, and April 1 in the same year. Each is 7 or 14 days from March 18, 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 +1.0% for 2000–2004, +2.5% for 2005–2009, and +0.7% 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 March 18 rank change over time?
Somewhat, but less than most dates. We ranked 365 recurring dates by SSA births in each five-year period. March 18 placed #303 in 2000–2004, #278 in 2005–2009, and #206 in 2010–2014. The three ranks span 97 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 March 18. It fell on a weekend twice in 2000–2004, twice in 2005–2009, and once in 2010–2014. 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 March guide for the full month ranking. Compare the nearby March 17 study.
The methodology explains ranks and shares. Use the birthday checker to compare another date.