The (Big) Difference Between One & Two


Having recently moved to the Bay Area, I am often asked about the differences in protocols for treating infertility at other centers across the United States. Simply put, the most notable variation comes down to the decision between one and two – embryos, that is.

I was fortunate enough to train at the University of Iowa where, in addition to learning a lot about corn and soybeans, I trained with the pioneers of the country’s first mandatory single embryo transfer policy. This meant that women meeting certain criteria would only receive a single embryo during their IVF cycle.  At first consideration, transferring only one embryo might seem a little crazy. This is immediately apparent when one reviews data on IVF outcome published on the CDC website: you will see that birth rates are higher when two embryos are transferred rather than one.

It is the goal of every fertility specialist in the United States to attain the highest pregnancy rates possible. So why would any program want to jeopardize their success? Well, the CDC data does not tell the whole story. If we take into account other variables such as age, previous experience with IVF, embryo quality and a clinic’s pregnancy rates, we can determine the best candidates for an elective single embryo transfer and greatly improve successful pregnancy rates.

Although Reproductive Science Center does not have a mandatory policy, we do follow SART guidelines in determining how many embryos to replace. This includes transferring only one embryo in a “good” candidate who is less than 35 years old.  However, success rates at RSC continue to be very high even in older women. The table below shows RSC’s 2012 pregnancy rates when transferring one and two embryos to women less than 38 years old or women using donor eggs:

1 Embryo Transferred 2 Embryos Transferred
<38 years old 70% 66% (60% Twins)
Egg donor recipient 74% 77% (90% Twins)


As you can see, pregnancy rates were essentially equivalent in these women whether they received one or two embryos. However, the twin rates in the women receiving two were between 60-90%. Well, I know what you are going to say… “I wouldn’t mind having twins!” As reproductive endocrinologists, we hear this each day. Although the majority of twin pregnancies are safe, the risk of adverse pregnancy outcomes is much higher for twin gestations. Remember, we not only want you to become pregnant, but we want you to have a safe pregnancy with the healthiest outcomes possible.

Although it is not mandatory at RSC, if you are offered single embryo transfer, congratulations are in order. Get excited because this means that you fall into the group with the highest success rate and the chances of soon becoming pregnant are very real.

Mother kissing little baby at home

Breaking Barriers, Building Families

Since 1983, we have pioneered fertility treatment for every kind of family. We want to help you achieve your dream of having a baby.

Request appointment