MA 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 RC1 (p-value)
MZ twins
<20 85 98 117 95 91 84 61 61 67 67  
20–24 769 669 650 655 635.5 596 447 506 515 473  
25–29 1956 2090 1945 1778 1622.5 1549 1238 1448 1408 1263  
30–34 1793 1663.5 1659 1840 1842 1778 1448.5 1712 1784 1736  
35–39 533 529.5 523 595 666 615 624 804 878 947  
≥ 40 63 67 63 85 87 103 74.5 121 105 161  
DZ twins
<20 34 30 32 26 28 32 51 28 22 26  
20–24 372 418 406 380 364 364 423 388 320 367  
25–29 2124 2128 2012 2116 1998 1842 1955 1584 1557 1458  
30–34 2540 2850 2880 3148 3262 3540 3711 3540 3208 2791  
35–39 962 1170 1238 1490 1678 1876 1850 1958 1986 1744  
≥ 40 98 124 128 144 184 216 290 192 261 236  
Overall twin pairs
<20 129 142 163 134 129 125 119 98 93 101  
20–24 1168 1113 1085 1069 1037.5 987 899 932 868 862  
25–29 4178 4314 4034 3980 3705.5 3468 3257 3102 3038 2769  
30–34 4423 4593.5 4620 5075 5195 5424 5268.5 5335 5082 4601.5  
35–39 1522 1732.5 1808 2127 2383 2550 2522 2831 2930 2744.5  
≥ 40 168 197 199 236 279 327 377.5 325 375 406  
MZ twinning rate per 1000 births
<20 3.55 3.80 4.32 3.47 3.66 3.63 3.00 3.14 3.66 3.64 –0.046 (0.274)
20–24 4.41 3.95 3.95 4.09 4.26 4.15 3.32 3.71 3.91 3.64 –0.065 (0.057)
25–29 4.02 4.35 4.23 4.09 4.01 4.10 3.57 4.23 4.26 3.90 –0.019 (0.462)
30–34 4.54 4.11 4.06 4.43 4.42 4.19 3.51 4.02 4.25 4.22 –0.032 (0.351)
35–39 4.42 4.05 3.97 4.39 4.62 3.97 3.95 4.57 4.58 4.61 0.039 (0.246)
≥ 40 3.99 3.95 3.69 4.67 4.46 4.92 3.40 5.11 3.93 5.40 0.099 (0.189)
DZ twinning rate per 1000 births
<20 1.42 1.16 1.18 0.95 1.13 1.38 2.50 1.44 1.20 1.41 0.039 (0.439)
20–24 2.13 2.47 2.46 2.37 2.44 2.54 3.14 2.84 2.43 2.82 0.062* (0.041)
25–29 4.37 4.43 4.38 4.87 4.94 4.87 5.64 4.62 4.71 4.50 0.041 (0.368)
30–34 6.43 7.04 7.06 7.59 7.82 8.34 8.99 8.31 7.63 6.78 0.111 (0.224)
35–39 7.97 8.94 9.40 11.00 11.64 12.10 11.70 11.14 10.36 8.49 0.157 (0.365)
≥ 40 6.21 7.31 7.50 7.91 9.43 10.31 13.22 8.11 9.77 7.91 0.318 (0.158)
Overall twinning rate per 1000 births
<20 5.39 5.50 6.02 4.89 5.19 5.39 5.84 5.05 5.09 5.48 –0.023 (0.577)
20–24 6.69 6.56 6.59 6.68 6.95 6.88 6.68 6.83 6.60 6.63 0.005 (0.752)
25–29 8.59 8.97 8.78 9.15 9.17 9.18 9.40 9.06 9.19 8.55 0.020 (0.548)
30–34 11.20 11.34 11.32 12.23 12.46 12.78 12.76 12.53 12.09 11.17 0.078 (0.309)
35–39 12.61 13.24 13.73 15.70 16.54 16.45 15.95 16.11 15.28 13.36 0.203 (0.238)
≥40 10.64 11.61 11.65 12.96 14.29 15.61 17.20 13.72 14.03 13.61 0.413* (0.047)
Linear regression coefficient (p–value) of twinning rate on the MA
Year 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008  
MZ twins 0.015 0.004 –0.019 0.042* 0.032* 0.035 0.022 0.072** 0.019 0.073**  
  (0.488) (0.698) (0.078) (0.012) (0.047) (0.089) (0.182) (0.004) (0.288) (0.003)  
DZ twins 0.258* 0.313** 0.327** 0.376* 0.429** 0.458** 0.495** 0.495** 0.415** 0.308**  
  (0.016) (0.009) (0.009) (0.016) (0.006) (0.004) (<0.001) (<0.001) (0.001) (0.002)  
Overall twins 0.276* 0.314** 0.311* 0.418* 0.462** 0.498** 0.527** 0.444** 0.439** 0.379**  
  (0.019) (0.009) (0.012) (0.010) (0.005) (0.001) (<0.001) (0.007) (0.002) (<0.001)  
*RC1: Regression coefficient of twinning rate on the year, *Significant at the 5% level, **Significant at the 1% level.
Table 2: Number of twin pairs and twinning rates by zygosity and overall twins by maternal age (MA), 1999-2008
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