Multiple pregnancy is a harm of treatment, not a bonus
Stimulation raises the chance of twins several-fold, and twins are the main source of avoidable harm in fertility care.
Couples are often quietly pleased at the idea of twins. Obstetricians are not, and the reason is not squeamishness.
From the network meta-analysis of unexplained infertility, in absolute terms: if the chance of multiple pregnancy following expectant management or IUI is assumed to be 0.6%, the chance following OS, OS-IUI, and IVF/ICSI would be 0.6% to 5.0%, 0.6% to 5.4%, and 0.4% to 5.5%.
Why that matters is visible in the national outcome data for babies conceived by assisted reproduction:
| Group, 2022 US ART births | Born before 37 weeks | Low birth weight |
|---|---|---|
| Single baby from a single-fetus pregnancy | 12.5% | 8.1% |
| Single baby from a multiple-fetus pregnancy | 21.9% | 16.8% |
| Twins | 63.2% | 50.8% |
| Triplets or more | 98.3% | 85.3% |
Nearly two thirds of twins are born early. Almost all triplets are. Prematurity is the single largest driver of newborn intensive care, of long stays, and of the outcomes nobody wants to discuss at a first fertility appointment.
This runs through the rest of this part. When you see an add-on that raises the multiple pregnancy rate, that is a harm counted against it, not a neutral side effect.