Risk of a neural tube defect in offspring

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

Reproduced with permission from: Nussbaum RL, McInnes RR, Willard HF. Genetics of disorders with complex inheritance. In: Thompson and Thompson Genetics in Medicine, 6th ed, WB Saunders, Philadelphia 2001. p. 289. Copyright © 2001 Elsevier.

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