Single or double embryo transfer: why most transfers use one embryo

By Berkcan K., editor · Published 2026-10-24

Most US IVF transfers now use one embryo. The CDC’s National ART Summary reports that in 2022, 85.9% of embryo transfers were single embryo transfers. Clinic data show what that means for births: nationally, for patients under 35 using their own eggs, 49.1% of transfers led to a live birth and 46.2% to a singleton birth, so fewer than 3 transfers in 100 ended in twins or more. Across the 372 clinics with at least 20 such transfers, the median clinic’s gap between the two rates was 2.3 points; 60 clinics had no gap at all, and 81 had a gap of 5 points or more.

What counts as a single embryo transfer?

The CDC glossary defines SET as “a procedure where one embryo, regardless of how many embryos are available, is transferred,” adding that the embryo “might be a frozen (cryopreserved) embryo from a previous IVF cycle or a fresh embryo from the current fresh IVF cycle.” ASRM uses the narrower term elective single-embryo transfer (eSET) for the case where a patient “chooses to have a single embryo transferred when multiple embryos are available.” The CDC’s 85.9% counts every transfer of one embryo, whether or not more were available.

The CDC states the purpose plainly: “The use of SET is a strategy to avoid a multiple-fetus pregnancy and reduce the risk of poor health outcomes, such as prematurity and low birth weight, among infants.” ASRM’s fact sheet says that “the primary goal of eSET is to decrease the multiple pregnancy rate associated with IVF.”

How often do clinics’ IVF births end in twins?

The CDC clinic report does not publish a twin rate, but it does print two rates on the same base: live-birth deliveries per transfer and singleton live-birth deliveries per transfer. The glossary defines a live-birth delivery as one or more infants with at least one born alive, a singleton delivery as “the delivery of a single infant born alive,” and a multiple live-birth delivery as “the delivery of more than one infant with at least one born alive.” Subtract the singleton rate from the live-birth rate and what is left is, roughly, the number of transfers in every hundred that ended in a multiple delivery.

Patients using their own eggs, CDC 2022 Final. National rates are the CDC’s; the clinic columns are calculated here from each clinic’s two CDC rates, for clinics with 20 or more transfers in the band among the 449 clinics with a page on this site.

Age band	National live births per transfer	National singleton births per transfer	Clinics with 20+ transfers	Median clinic gap	Clinics with no gap	Clinics with a gap of 5+ points
Under 35	49.1%	46.2%	372	2.3 points	60	81
35 to 37	46.0%	43.7%	323	1.9 points	101	41
38 to 40	41.2%	39.2%	267	1.4 points	107	23
Over 40	25.4%	24.5%	139	0.0 points	90	6

The gap shrinks with age: over 40, 90 of the 139 clinics had no gap at all, and only 6 had a gap of 5 points or more. The under-35 band is where clinics differ most: about one clinic in five (81 of 372) had at least 5 transfers in every hundred end in a multiple delivery, against a national figure under 3.

A gap is not a direct count of how many embryos a clinic transferred. Even single transfers produce some twins: ASRM puts twin rates after eSET at “around 1%-2%”. A larger gap is the signal worth asking about, since, in ASRM’s words, “transferring more than one embryo increases the chance of having a multiple pregnancy.”

Do states differ?

Among states with eight or more qualifying clinics, the share of under-35 clinics with a gap of 5 points or more ranges widely:

California: 17 of 56 clinics.
Texas: 9 of 37.
New York: 5 of 36.
Florida: 5 of 22.
Virginia and Pennsylvania: 0 of 9 each.

With so few clinics per state, this is a description of practice in 2022, not a ranking. Each clinic page shows its live-birth and singleton rows side by side for every band, so the gap can be read directly.

Does transferring one embryo lower the chance of a baby?

ASRM’s fact sheet says there is “a small decrease in overall pregnancy rates after eSET”, partly because it is sometimes offered to patients who are not good candidates, but that “freezing extra embryos and transferring in later cycles can give a comparable pregnancy rate without the risks associated with twinning.” The CDC’s cumulative rates per intended retrieval capture exactly that, because they count every transfer within a year of the retrieval, frozen ones included.

ASRM also lists the risks a multiple pregnancy carries, including preterm delivery, “maternal hemorrhage (severe bleeding)”, C-section, high blood pressure and gestational diabetes, and says that “the more embryos that are transferred into the uterus, the greater the risk.” Its IVF fact sheet tells patients that “your doctor should transfer the minimum number of embryos necessary to provide a high likelihood of pregnancy with the lowest risk of multiple pregnancy.” How many embryos are right in a particular case is a decision for the patient and the doctor; ASRM notes that the recommendation “is usually based on her age, history and how many embryos there are available to freeze.”

What to ask a clinic about embryo numbers

What is your policy on single-embryo transfer for patients like me? The cost questions guide covers how that policy affects the bill for later frozen transfers.
How does your singleton rate compare with your overall rate in my band? A gap well above the national figure deserves an explanation.
How many embryos are typically frozen after a retrieval? That determines whether a single transfer leaves options for later.

The comparison tool shows the live-birth and singleton rows for two or three clinics side by side, with the number of transfers behind each.

Frozen vs fresh embryo transfer: why most single transfers today use a frozen embryo.
How to read CDC IVF success rates: the singleton rows and the cumulative rates in context.
What the IVF age bands mean: the national rates by age that these gaps sit beside.