FERTILISATION RATE

Fertilisation rate proposed as a strong indicator of cumulative live birth rate

Published 26 July 2021

A retrospective study of almost 10,000 ICSI cycles from the Italian national registry suggests that fertilisation rate may predict cumulative LBR as strongly as female age or the number of eggs retrieved.

Fertilisation rate has been traditionally used as a laboratory performance indicator but now the authors of a new study suggest that it can also be used as a strong predictor of clinical outcome.

This multicentre retrospective cohort study analysed the data of 9394 ICSI cycles in the Italian National ART Registry.(1) Only patients with a completed ICSI treatment, including frozen embryo transfers, were included in the study. The cumulative LBR per treatment cycle was primarily measured in terms of three fertilisation groups: <65% defined as poor; 65-80% as competent; and >80% as benchmark as described in the Vienna consensus on laboratory performance indicators.(2) The fertilisation rate groups were also compared for cumulative LBR when maternal age (<34, 35-38, 39-42 years) and number of retrieved oocytes (5-7, 8-10, >10 oocytes) were included in the analysis.
According to the study results, fertilisation rate was strongly and positively correlated with cumulative LBR in the overall analysis, and this association was also present in the subgroup analysis according to maternal age: in patients aged under 34 years the cumulative LBR increased significantly with increasing levels of fertilisation: from 29% to 48.8% to 55.6%; in the 35–38 years group from 22% to 36.9% to 42.3% in the three fertilisation rate groups; and in older patients (39–42 years) from 10% to 18.8% and to 25.2% in the group with the highest fertilisation rate.

Additionally, when the number of retrieved oocytes was considered, fertilisation rate was similarly associated with cumulative LBR in the three categories (5-7, 8-10 and >10 oocytes). When both maternal age and number of oocytes were included in the analysis, the fertilisation rate was also positively correlated with cumulative LBR when fewer oocytes were collected (5-7); this was evident in all age categories – although in younger patients with a large number of oocytes collected (>10), cumulative LBR was not associated with fertilisation rate.

It has been traditionally proposed that, among other indicators, the number of oocytes retrieved during egg collection is a strong predictor of clinical outcome.(3,4) Polyzos et al confirmed that finding and clearly showed that cumulative LBR increases with the number of oocytes retrieved, even when a high number were retrieved. However, the landmark study of Sunkara et al, while indicating that LBR does indeed correlate positively with the number of oocytes retrieved in all age groups, did also show that LBR decreases when more than 20 oocytes were collected.

Now, according to these Italian authors, fertilisation rate appears to be a more comprehensive indicator of cumulative LBR since it takes into account the effect of sperm and embryo development. The findings of this large study thus indicate that cumulative LBR is positively correlated with fertilisation rate, which itself probably derives from the higher number of embryos available for transfer.

However, in younger patients with many oocytes available for fertilisation, this association dissipates. The authors suggest that, cumulatively, the high number and the good quality of oocytes of younger patients have a distinctive role on LBR.

So should the fertilisation rate in ICSI cycles be considered an independent performance indicator of clinical outcome in the same way as maternal age or the number of oocytes retrieved? The authors of this study clearly think ‘yes’, and further analysis of a large dataset would probably confirm the statement.


1. Scaravelli G, Zacà C, Setti PEL, et al. Fertilization rate as a novel indicator for cumulative live birth rate: a multicenter retrospective cohort study of 9,394 complete in vitro fertilization cycles. Fertil Steril 2021; doi:10.1016/j.fertnstert.2021.04.006.
2. The Vienna consensus: report of an expert meeting on the development of ART laboratory performance indicators. Reprod Biomed Online 2017; 35: 494-510. doi:10.1016/j.rbmo.2017.06.015.
3. Polyzos NP, Drakopoulos P, Parra J, et al. Cumulative live birth rates according to the number of oocytes retrieved after the first ovarian stimulation for in vitro fertilization/intracytoplasmic sperm injection: a multicenter multinational analysis including ∼15,000 women. Fertil Steril 2018; 110: 661-670.e1.
doi: 10.1016/j.fertnstert.2018.04.039.
4. Sunkara SK, Rittenberg V, Raine-Fenning N, et al. Association between the number of eggs and live birth in IVF treatment: an analysis of 400 135 treatment cycles. Hum Reprod 2011; 26: 1768–1774.
doi:10.1093/humrep/der106.

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