The father's age
The effect is real, smaller than the mother's, and inconsistently found for time to conception.
The evidence here genuinely disagrees with itself, and this guide will say so rather than pick the tidier answer.
On time to conception, the studies conflict. The European daily-records study found that from the late thirties male age was an important factor, with the percentage of couples failing to conceive within twelve cycles rising from about 18% to 28% as the man moved from 35 to 40. The North American cohort found male age was not associated appreciably with fecundability after adjusting for female age, and said outright that it had too few men over 45 to conclude anything about them. The fertility societies write that male fertility does not appear to be appreciably affected before the age of approximately 50.
On miscarriage, the evidence is consistent and modest. Nine studies pooled, all adjusted at minimum for the mother's age, compared with fathers aged 25 to 29:
| Father's age | Pooled relative risk of miscarriage |
|---|---|
| 30 to 34 | 1.04 (95% CI 0.90 to 1.21) |
| 35 to 39 | 1.15 (95% CI 0.92 to 1.43) |
| 40 to 44 | 1.23 (95% CI 1.06 to 1.43) |
| 45 and over | 1.43 (95% CI 1.13 to 1.81) |
The authors of that review add their own caution: the paternal age effect is less pronounced than that observed with advanced maternal age and residual confounding by maternal age cannot be excluded.
On the pregnancy itself, a study of 40.5 million US births found that fathers aged 45 and over, compared with 25 to 34 and adjusted for the mother's age, had a slightly higher chance of premature birth (odds ratio 1.14, 95% CI 1.13 to 1.15) and of gestational diabetes in the mother (1.34, 95% CI 1.29 to 1.38). The authors' own summary of the magnitude: Given the relatively low prevalence of advanced paternal age in the US, population level impacts are currently modest.