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Zona Pellucida Thickness Not a Predictor of Pregnancy Outcomes: Presented at ASRM

By Crina Frincu-Mallos, PhD

WASHINGTON, DC -- October 24, 2007 -- A thickened zona pellucida is a frequently used yet poorly defined predictor of pregnancy outcome in patients undergoing in vitro fertilization (IVF) with assisted hatching.

However, the results of a prospective study on assisted hatching clearly indicate that the concept of thickened zona pellucida is irrelevant in determining whether assisted hatching has clinical benefit or not, researchers reported here at the 63rd Annual Meeting of the American Society for Reproductive Medicine (ASRM).

Lead author Andrea R. Hagemann, MD, Chief Resident, Division of Reproductive Endocrinology and Infertility, Washington University School of Medicine, Saint Louis, Missouri, United States, and colleagues described the results here on October 15.

Dr. Hageman and colleagues aimed to determine the relationship between zone pellucida thickness and clinical pregnancy rates in patients less than 38 years of age undergoing IVF. A secondary objective was to define the zona pellucida thickness at which assisted hatching is beneficial, to provide an evidence-based definition of the concept of thickened zona pellucida, explained the researchers.

This was a randomised, controlled, double-blind study of assisted hatching that enrolled 162 patients between April 20, 2004, and February 1, 2007, said Dr. Hagemann and colleagues.

Each patient entered the study on or before the day of egg retrieval; and either were scheduled for a day 3 embryo transfer (n=55 IVF cycles), a day 5 transfer, or failed fertilisation.

Only participants scheduled for a day 3 embryo transfer had the thickness of each zona pellucida measured, using the Zona Laser Treatment System (Hamilton Thorne, Beverly, MA). The rest of the patients were terminated from the trial.

A total of 98 measurements from 55 IVF cycles were available for comparison.

A total of 87 patients with embryos having a zona pellucida thickness of 13 mcm or greater were randomised to undergo assisted hatching or to remain unhatched. Eleven patients whose embryos had a zona pellucida thickness less than 13 mcm were labelled as "thin" and excluded from randomisation.

All assisted hatching procedures were performed on the morning of day 3, at least 1 hour prior to the scheduled embryo transfer, using acidic Tyrode's medium (Irvine Scientific, CA).

Statistical evaluations were performed using an unpaired two-tailed t-test. A log-binomial model was used to generate a receiver operator curve (ROC).

In the group of patients randomised to IVF without assisted hatching, there was no statistically significant difference (P =.28) in mean zona pellucida thickness between those with positive pregnancy outcomes (17.6, n=8) compared with those who had negative outcomes (16.8, n=5), said the researchers.

A ROC showed a 1-unit increase in mean zona pellucida thickness after adjusting for assisted hatching. This result correlated with a small increase in the probability of pregnancy, but the difference is not statistically significant (RR=1.11, 95% CI 0.96-1.28), noted Dr. Hagemann and colleagues.

Zona pellucida thickness was not a significant determinant of clinical pregnancy in this patient population undergoing IVF without assisted hatching, according to the researchers.

Furthermore, there does not appear to be a threshold of zona pellucida thickness where assisted hatching would provide a clinical benefit, speculated the researchers.

Dr. Hagemann and colleagues suggested that the concept of thickened zona pellucida is irrelevant and hatching should not be offered to this patient population.

Funding for this study was provided by Barnes-Jewish Hospital Foundation.


[Presentation title: Zona Pellucida Thickness Does Not Affect Clinical Pregnancy Rates in Patients less than 38 Years of Age Undergoing in Vitro Fertilization: Results from a Prospective, Randomized Trial on Assisted Hatching. Abstract P-67]

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