The biggest single driver of the modern twin boom
Twin birth rates in high-income countries roughly doubled between 1980 and 2010 — and that rise has nothing to do with genetics. Kulkarni and colleagues, using CDC ART surveillance data, attributed roughly two-thirds of the increase in twin births in the United States to assisted reproductive technology and ovulation-stimulating medication [4]. The rest is largely explained by later maternal age.
Two distinct mechanisms are at play in fertility care:
- Multi-embryo transfer in IVF. Transferring more than one embryo increases the chance that at least one implants — at the cost of increasing the chance that more than one does.
- Superovulation. Drugs like clomiphene citrate, letrozole and gonadotropins recruit multiple follicles in a single cycle, raising the chance that two eggs are released and both fertilised — even outside an IVF cycle (in IUI or natural-cycle attempts).
The numbers, by treatment type
The CDC’s most recent ART Surveillance Report shows that among IVF cycles using fresh embryos transferred in 2022, the live-birth twinning rate sat near 7%, down from 31% in 2009 following the wide adoption of single-embryo transfer (SET) [1]. Modern numbers vary sharply with the protocol:
- Single-embryo transfer (SET): twin rates close to spontaneous conception (~2–3%)
- Double-embryo transfer (DET): twin rates of 25–35% in younger women
- IUI with gonadotropin stimulation: twin rates of 15–25%, and triplet rates that can exceed 5%
- Clomiphene without monitoring: twin rates around 5–8%
For the multiplicative model behind this calculator, we apply a uniform ×3.5 multiplier when any ART pathway is reported. Future versions will distinguish SET from DET and ovulation induction from IVF.
Why ASRM moved aggressively to SET
By the early 2000s, twin and higher-order multiple pregnancies from IVF were producing measurable population-level increases in preterm birth, low birth weight, NICU admissions and obstetric complications. The American Society for Reproductive Medicine and ACOG responded by progressively tightening the recommended number of embryos to transfer, especially in younger women with good prognosis [2][3].
The result is striking: while the absolute number of IVF cycles in the US continues to rise, the share resulting in twin births has fallen dramatically. ART is no longer the volcano it was for twin rates.
What it means for your decision
If you are mid-treatment and weighing single vs. double-embryo transfer, the trade-off is concrete: SET typically gives a slightly lower per-transfer pregnancy rate but a markedly safer obstetric outcome distribution. ASRM’s published guidance includes age-specific recommendations and is worth reviewing with your clinic — patients are entitled to the same evidence base their physicians use.
If you are TTC and not in treatment, this factor does not apply to you. Move on to family history or maternal age.