Unexplained differences in sex ratios at birth in Europe and North America

In mammals, male live births exceed female ones. In humans, the ratio of male births to total births is expected to be 0.515. In Europe during 1990-5 this differed significantly with increasing geographical latitude.¹ We analysed and compared the male to female ratio in Europe and North America over 50 years.

Methods and results

We obtained annual data on male and female live births from the World Health Organization (WHO) for the North American continent for 1958-97 and for European countries for 1950-99. Overall <3% of data were missing.

European countries were banded by latitude. Southern countries (latitude 35-40°) included Bulgaria, Greece, Italy, Malta, Portugal, and Spain. Central Europe (40-55°) included Austria, Belgium, Czech Republic, France, Germany, Hungary, Ireland, Luxembourg, Netherlands, Poland, Romania, Switzerland, and the United Kingdom. Nordic countries (>55°) include Denmark, Finland, Iceland, Norway, and Sweden. North America was divided by latitude into Canada (>50°), the United States (30-50°), and Mexico (<30°).

We analysed contingency tables using χ² and χ² for trend and obtained 95% confidence intervals for ratios by using the Fleiss equations. P⩽0.05 was taken as significant.

Results

Significantly more boys were born in southern countries (table) than in central Europe (χ²=57, P<0.0001) or the Nordic countries (χ²=8.8, P=0.003; χ² for trend=120, P<0.0001). The difference between central Europe and the Nordic countries was not significant. All had a male to female ratio <0.515, with a resultant male birth deficit of 12 744 in the Mediterranean, 212 780 in central Europe, and 13 169 in the Nordic countries (total deficit of male births 238 693).

A low male to female ratio was found in Mexico, a higher ratio in the United States, and an even higher ratio in Canada (χ² for trend=57, P<0.0001). All had a male to female ratio <0.515, with a resultant male birth deficit of 21 993 in Canada, 410 932 in the United States, and 521 789 in Mexico (total deficit 954 714).

In the two continents, the total male birth deficit was 1 193 407 live births.

Central Europe and the southern countries span latitudes equivalent to the United States. The male to female ratio for the total number of births of these two European regions was significantly higher than that for the United States (χ²=499, P<0.0001).

The Nordic countries span latitudes equivalent to Canada. Although the male to female ratio in these countries was higher than in Canada, this difference was not significant.

Comment

In Europe, significantly more male babies were born in southern latitudes than in northern latitudes, whereas the reverse was found in North America. We are unable to explain these findings, which do not support a temperature related effect.

Table.

Numbers of male, female, and total births, with calculated male:female ratios for countries included in study of sex ratios at birth in Europe and North America

Region/country	Live births
	Male	Female	Total	Male:female ratio (95% CI)
Nordic countries (>55°):	7 579 510	7 163 556	14 743 066	0.5141 (0.5139 to 0.5144)
Denmark	1 680 922	1 652 473	3 394 155	0.5141 (0.5136 to 0.5147)
Finland	1 741 682	1 652 473	3 394 155	0.5131 (0.5126 to 0.5137)
Iceland	104 747	98 862	203 609	0.5145 (0.5123 to 0.5166)
Norway	1 444 600	1 363 935	2 808 535	0.5144 (0.5138 to 0.5149)
Sweden	2 607 559	2 459 796	5 067 355	0.5146 (0.5141 to 0.5150)
Southern countries (35-40°):	43 290 341	40 793 319	84 083 660	0.5148 (0.5147 to 0.5150)
Bulgaria	3 176 760	3 000 118	6 176 878	0.5143 (0.5139 to 0.5147)
Greece	3 395 944	3 168 572	6 564 516	0.5173 (0.5169 to 0.5177)
Malta	162 065	151 445	313 510	0.5169 (0.5152 to 0.5187)
Portugal	4 283 717	4 015 694	8 299 411	0.5161 (0.5158 to 0.5165)
Italy	18 671 513	17 652 250	36 323 763	0.5140 (0.5139 to 0.5142)
Spain	13 600 342	12 805 240	26 405 582	0.5151 (0.5149 to 0.5152)
Central Europe (40-55°):	108 325 130	102 428 093	210 753 223	0.5140 (0.5139 to 0.5141)
Austria	2 600 186	2 466 512	5 066 698	0.5132 (0.5128 to 0.5136)
Belgium	3 138 073	2 965 932	6 104 005	0.5141 (0.5137 to 0.5145)
Czech Republic	817 297	774 436	1 591 733	0.5135 (0.5127 to 0.5142)
France	19 579 581	18 634 567	38 214 148	0.5124 (0.5122 to 0.5125)
Hungary	3 815 036	3 587 994	7 403 030	0.5153 (0.5150 to 0.5157)
Ireland	1 498 481	1 416 062	2 914 543	0.5141 (0.5136 to 0.5147)
Luxembourg	114 859	108 397	223 256	0.5145 (0.5124 to 0.5165)
Netherlands	5 160 590	4 899 973	10 060 563	0.5130 (0.5126 to 0.5133)
Poland	15 295 234	14 359 427	29 654 661	0.5158 (0.5156 to 0.5160)
Romania	8 082 232	7 649 545	15 731 777	0.5138 (0.5135 to 0.5140)
Switzerland	2 103 563	1 995 775	4 099 338	0.5131 (0.5127 to 0.5136)
United Kingdom	20 441 737	19 330 020	39 771 757	0.5140 (0.5138 to 0.5141)
Germany	25 678 261	24 239 453	49 917 714	0.5144 (0.5143 to 0.5146)
Central Europe and Mediterranean (35-55°)	151 615 471	143 221 412	294 836 883	0.5142 (0.5142 to 0.5143)
All Europe	159 194 981	150 384 968	309 579 949	0.5142 (0.5142 to 0.5143)
North America:	127 034 732	121 488 464	248 523 196	0.5112 (0.5111 to 0.5112)
United States (30-50°)	76 827 922	73 150 435	149 978 357	0.5123 (0.5122 to 0.5123)
Mexico (<30°)	42 193 928	40 749 212	82 943 140	0.5087 (0.5086 to 0.5088)

Data for the United States and Mexico include 1958-97. Data for all European countries include 1950-99. The male:female ratio was calculated as male live births divided by total live births.