| Affected relatives | Risk of anencephaly and spina bifida (percent) |
| No siblings | |
| Neither parent | 0.3 |
| One parent | 4.5 |
| Both parents | 30 |
| One sibling | |
| Neither parent | 4 |
| One parent | 12 |
| Both parents | 38 |
| Two siblings | |
| Neither parent | 10 |
| One parent | 20 |
| Both parents | 43 |
| One sibling and one second-degree relative | |
| Neither parent | 7 |
| One parent | 18 |
| Both parents | 42 |
| One sibling and one third-degree relative | |
| Neither parent | 5.5 |
| One parent | 16 |
| Both parents | 42 |
