SINGLE EMBRYO REPLACEMENT FOR IVF

The very early IVF pregnancies in the late 1970’s, were conceived from natural cycles, but the pregnancy rates (i.e. babies produced) per attempts were very poor. The pregnancy rate increased with the replacement of multiple embryos following ovulation stimulation with drugs. Some units still use natural cycle IVF, mainly for philosophical reasons, but although success rates are better than those from earlier days they do not match those from ovulation stimulation and multiple embryo replacement.

With improvements in culture systems, equipment and techniques the success rates continue to get better and in general, embryo quality continues to improve.

The problem with multiple embryo replacement is, of course, multiple pregnancies. Most experienced Fertility Centres replace two embryos for the large majority of couples and the risk of twins is once every five pregnancies (actual 22%). This rate has not changed despite reducing replacement numbers from three to two. However in women of less than 35 years the multiple pregnancy rate is over 30%.

We do not know the success rates of replacing only one embryo after having collected and fertilised many oocytes (eggs), and presumably cryopreserved (frozen) the surplus embryos, although we believe that the implantation rate for each embryo will not change with the replacement of one embryo. By replacing one embryo, we can eliminate the multiple pregnancy rate and yet we believe still produce the same number of conceptions from each oocyte collection.

Twin pregnancies have problems. They have almost twice the stillbirth and early death rate as single births. There are more cerebral palsy children from twin pregnancies. There are more congenital abnormalities in twins. The late miscarriage and early (premature) birth rate is higher. The total figures of risk are not high but they are very much higher than for single births.

It seems appropriate to offer couples the choice of replacing a single embryo, admitting that it is likely the chance of conception per replacement will be less but the chances of multiple pregnancies are virtually eliminated. With the freezing of surplus embryos there will be an increase in the number of replacements from each egg collection.