ABSTRACT
Age-corrected implantation rates with the use of assisted reproductive technologies (ART) have been on the increase over the past 25 years. Despite this success, supernumerary embryos are still transferred simultaneously, with the risk of increased multiple pregnancy rates. Although enforced restrictive policies of single embryo transfer have yet to be enacted in most countries, the desire to select the best embryos and predict implantation has aided the development of several evaluation methods to enhance selection of embryos with optimal prognosis for implantation. Embryo selection is most commonly based on morphological criteria that are assessed postfertilization, including both pronuclear and cleavage stage embryo evaluation. Studies of phenomena related to development rate, pronuclear and nucleolar behavior, blastomere fragmentation, and multinucleation have been of particular interest.1-8
Events related to pronuclear and nucleolar movements were first described by Wright et al2 and Van Blerkom.9 Recently, these phenomena have been expressed as more distinct pronuclear scoring systems that can be used for the purpose of selecting embryos. The scores have been correlated with improved embryo development5,10-13 as well as with increased pregnancy and implantation rates.5,10,12-20
A number of different pronuclear scoring systems have been proposed by different ART laboratories, and are in use to select high quality embryos; however, no standardized scoring system for zygote grading is currently in use. Unfortunately, the same holds true for other systems used to evaluate embryo morphology. As a result, comparing success rates between laboratories while controlling for embryo quality is a challenge. A consistent scoring system, such as a
pronuclear scoring system in combination with other embryo development markers and patient status, may be useful in determining the optimal number of embryos to transfer. This could lead to an increase in pregnancy and implantation rates, while decreasing the number of high order multiple pregnancies.
