Table 3.
Numbers and percentages of twin pairs for the different zygosity groups at 8 months and at 29 months
| 8 months | 29 months | |||
|---|---|---|---|---|
| Frequency (n) | Percent (%) | Frequency (n) | Percent (%) | |
| Sample of same-sex twin pairs (excluding unknown zygosity) | ||||
| Total | 1549 | 1257 | ||
| MZ | 749 | 48.4 | 616 | 49.1 |
| DZ | 800 | 51.6 | 641 | 50.1 |
| Zygosity groups according to parents’ beliefs of zygosity and zygosity derived from questionnaire and DNA data | ||||
| Total | 1528a | 100 | 898b | |
| MZC | 511 | 33.4 | 334 | 37.2 |
| MZI | 220 | 14.4 | 119 | 13.2 |
| DZC | 791 | 51.8 | 443 | 49.3 |
| DZI | 6 | 0.4 | 2 | 0.2 |
MZ Monozygotic; DZ Dizygotic; MZC MZ pairs correctly classified as MZ by parents; MZI MZ pairs misclassified as DZs by parents; DZC DZ pairs correctly classified as DZ by parents; DZI DZ pairs misclassified as MZs by parents
an is less than the total n for MZs (1549) because it only includes pairs with both classified zygosity at 8 months and pairs whose parents answered the question “do you think your twins are identical?”
bn is less than the total n for DZs (1257) because it only includes pairs with both classified zygosity at 29 months (using questionnaire and DNA data) and pairs whose parents answered the question “do you think your twins are identical?”