Table 1.

Parental age versus birth order

	Birth order			Total
	First	Middle	Last
A
30 years or younger	95	45	54	194
31–40 years	27	41	78	146
Older than 40 years	5	7	21	33
Total	127	93	153	373
B
25 years or younger	57	32	20	109
26–35 years	43	45	93	181
Older than 35 years	2	10	24	36
Total	102	87	137	326

Categorized (A) paternal age and (B) maternal age versus birth order. χ²-tests for youngest versus oldest categories and first versus last born (gray cells, thus reducing to a 2 × 2 table) suggest a birth-order effect in both tables (A: P = 1.92 × 10^−4, B: P = 1.32 × 10⁻⁸).