But now I have the chance to address the issue of "fathering twins" head on. Namely, if you want to father twins, what if anything can you do to increase your chances? The question breaks down separately for fraternal and identical twins.
Most sources agree on what increases the probability of fraternal twins appearing. For example, Twins! Pregnancy, Birth and the First Year of Life lists four relevant factors:
- Race (A 1999 paper mentions that the highest rates are found in Nigeria; the lowest in Japan.)
- Mother's Age and Obstetric History (For example, a 1967 study found the rate to more than triple for 40-year-old mothers versus 20-year-old mothers. The same study found the rate to similarly increase in the incidence of twins as the number of a mother's pregnancies increased.)
- Genetics (This is the source of the common question, "Do twins run in your family?")
- Infertility Treatments (The implantation of multiple embryos in IVF pushes the fraternal twinning rate as high as 30%; fertility drugs which stimulate ovulation are responsible for, among other things, Jon and Kate Plus 8.)
Many sources agree about what influences the probability of identical twins: nothing. Those sources, however, are wrong. For example, multiples.about.com says
The rate for identical, or monozygotic, multiples is random and universal; it's the same in all populations regardless of race, heredity or other factors, and it has remained constant over time. The chances of having identical twins is about 1 in 285.In fact, there are potential "other factors" that make a difference, although the research is far from settled. There's some evidence that improved nutrition helps. The National Geographic special In the Womb: Identical Twins claims that maternal age plays a role (women who are at either end of their reproductive years are more likely to have identical twins), although I've found this claim in no other source.
One consistent theme seems to be that IVF has a role in increasing the rate of identical twins, although nobody can agree on how much. Liza Mundy in Everything Conceivable claims that
For IVF embryos, the rate of monozygotic twinning is ten times that of naturally conceived pregnancies.As this was a far higher rate than I had seen claimed elsewhere, I asked Ms. Mundy via e-mail what the source of the data was. It appears that it is either "the NICHD fertility outcomes conference" or the book Iatrogenic Multiple Pregnancy: Clinical Implications. Thanks to Amazon's "search inside" feature, I see a claim in the latter that monozygotic (identical) twinning is 12 times more likely under IVF. In one particular study, 4 out of 85 IVF pregnancies resulted in "MZ" twinning, when the "natural" rate would have been 1/3 of a pregnancy. Hence, 12 times. Unfortunately, the authors of that study do not seem to have been able to use a statistical significance calculator. A quick Google search produces one with the verdict: No.
A more recent study, however, with a larger sample size, found a 2% rate of monozygotic twinning. (Unfortunately, that rate was based on early ultrasounds and did not take into account the sadly high incidence of "vanishing twin syndrome".) Attempts were made to ascribe the increased incidence to various factors related to the IVF process, with mixed success.
Another article looks at the various studies says, Monozygotic twinning occurs more frequently (∼1.5–2.0%) after IVF/ICSI than after natural conception (0.42%) but notes that there have been contradictory studies about more detailed risk factors. That's probably the best we can say for now.
Trying (and failing) to get to the bottom of this took more work than I expected, but here's my advice: if you want to father identical twins, you may or may not want to seek out a younger or older mate, you probably want to look into IVF, but you definitely want to make sure she eats a healthy diet. She's going to need to be in the best shape possible to chase after the twins!