Cochrane review authors reviewed the evidence for culturing human embryos at different temperatures during assisted reproduction, to analyse which strategy leads to the highest live birth rate.
'Infertility' is defined as the failure to achieve pregnancy after 12 months or more of regular unprotected sexual intercourse. In vitro fertilisation (IVF) is one of the assisted reproductive techniques that can help infertile couples to have a baby. During IVF, multiple oocytes (eggs) are retrieved from the ovaries, and are fertilised by culturing them in a dish together with a sample of sperm, or by injecting a single sperm cell directly into the oocyte (intracytoplasmic sperm injection; ICSI). Before being transferred into the uterus the fertilised oocyte (known as an embryo), must be cultured in an incubator for several days for further development. During this incubation period, the embryo is usually cultured at a temperature of 37°C, to mimic human core body temperature. However, several studies have shown that the temperature inside the female reproductive tract may be lower than 37°C, suggesting that a lower incubator temperature might be more beneficial for embryo development. In this review we assessed different temperatures of embryo culture, which may lead to a higher live birth rate.
We included three randomised controlled trials that compared 37.0°C or 37.1°C with a lower incubator temperature. The studies took place in the USA, Belgium and Egypt and included a total of 563 women who all underwent IVF/ICSI. One study reported the live birth, comparing incubation of embryos at 37.0°C with 36.0°C, one study reported the clinical pregnancy, comparing incubation at 37.1°C with 36.6°C, and one study reported multiple outcomes (miscarriage, clinical pregnancy, ongoing pregnancy and multiple pregnancy), comparing incubation of embryos at 37.0°C with 36.5°C. Two studies reported no study funding or competing interests, the other study reported no information about funding or competing interests. The evidence is current to March 2019.
Only one study reported the primary outcome live birth, but due to a small sample size, randomisation on oocytes and paired design, no conclusions could be made. We are uncertain if incubating at a lower temperature than 37°C is beneficial for the following outcomes; miscarriage, clinical pregnancy, ongoing pregnancy and multiple pregnancy. Looking at clinical pregnancy, if women have a 55% chance of a clinical pregnancy with culturing embryos at 37°C, the clinical pregnancy rate using a lower temperature would be between 47% and 66%. Adverse events were mostly not reported; only one study reported no adverse events. Because the number of studies was limited, and each study reported different outcomes, more randomised controlled trials (RCTs) are needed in this field.
Quality of evidence
The quality of the evidence (using GRADE) was very low due to high risk of bias (performance bias) and imprecision (limited amount of studies and wide confidence intervals). Based on the limited and very low-quality included studies, there is no evidence that a lower temperature may enhance live birth rates or any of the other studied outcomes.
This review evaluated different temperatures for embryo culture during IVF. There is a lack of evidence for the majority of outcomes in this review. Based on very low-quality evidence, we are uncertain if incubating at a lower temperature than 37°C improves pregnancy outcomes. More RCTs are needed for comparing different temperatures of embryo culture which require reporting of clinical outcomes as live birth, miscarriage, clinical pregnancy and adverse events.
'Infertility' is defined as the failure to achieve pregnancy after 12 months or more of regular unprotected sexual intercourse. One in six couples experience a delay in becoming pregnant. In vitro fertilisation (IVF) is one of the assisted reproductive techniques used to enable couples to achieve a live birth. One of the processes involved in IVF is embryo culture in an incubator, where a stable environment is created and maintained. The incubators are set at approximately 37°C, which is based on the human core body temperature, although several studies have shown that this temperature may in fact be lower in the female reproductive tract and that this could be beneficial. In this review we have included randomised controlled trials which compared different temperatures of embryo culture.
To assess different temperatures of embryo culture for human assisted reproduction, which may lead to higher live birth rates.
We searched the following databases and trial registers: the Cochrane Gynaecology and Fertility (CGF) Group Specialised Register of Controlled Trials, the Cochrane Central Register of Studies Online, MEDLINE, Embase, PsycINFO, CINAHL, clinicaltrials.gov, The World Health Organization International Trials Registry Platform search portal, DARE, Web of Knowledge, OpenGrey, LILACS database, PubMed and Google Scholar. Furthermore, we manually searched the references of relevant articles and contacted experts in the field to obtain additional data. We did not restrict the search by language or publication status. We performed the last search on 6 March 2019.
Two review authors independently screened the titles and abstracts of articles retrieved by the search. Full texts of potentially eligible randomised controlled trials (RCTs) were obtained and screened. We included all RCTs which compared different temperatures of embryo culture in IVF or intracytoplasmic sperm injection (ICSI), with a minimum difference in temperature between the two incubators of ≥ 0.5°C. The search process is shown in the PRISMA flow chart.
Two review authors independently assessed trial eligibility and risk of bias and extracted data from the included studies; the third review author resolved any disagreements. We contacted trial authors to provide additional data. The primary review outcomes were live birth and miscarriage. Clinical pregnancy, ongoing pregnancy, multiple pregnancy and adverse events were secondary outcomes. All extracted data were dichotomous outcomes, and odds ratios (OR) were calculated with 95% confidence intervals (CIs) on an intention-to-treat basis. We assessed the overall quality of the evidence for the main comparisons using GRADE methods.
We included three RCTs, with a total of 563 women, that compared incubation of embryos at 37.0°C or 37.1°C with a lower incubator temperature (37.0°C versus 36.6°C, 37.1°C versus 36.0°C, 37.0° versus 36.5°C). Live birth, miscarriage, clinical pregnancy, ongoing pregnancy and multiple pregnancy were reported. After additional information from the authors, we confirmed one study as having no adverse events; the other two studies did not report adverse events. We did not perform a meta-analysis as there were not enough studies included per outcome. Live birth was not graded since there were no data of interest available. The evidence for the primary outcome, miscarriage, was of very low quality. The evidence for the secondary outcomes, clinical pregnancy, ongoing pregnancy and multiple pregnancy was also of very low quality. We downgraded the evidence because of high risk of bias (for performance bias) and imprecision due to limited included studies and wide CIs.
Only one study reported the primary outcome, live birth (n = 52). They performed randomisation at the level of oocytes and not per woman, and used a paired design whereby two embryos, one from 36.0°C and one from 37.0°C, were transferred. The data from this study were not interpretable in a meaningful way and therefore not presented. Only one study reported miscarriage. We are uncertain whether incubation at a lower temperature decreases the miscarriage (odds ratio (OR) 0.90, 95% CI 0.52 to 1.55; 1 study, N = 412; very low-quality evidence).
Of the two studies that reported clinical pregnancy, only one of them performed randomisation per woman. We are uncertain whether a lower temperature improves clinical pregnancy compared to 37°C for embryo incubation (OR 1.08, 95% CI 0.73 to 1.60; 1 study, N = 412; very low-quality evidence). For the outcome, ongoing pregnancy, we are uncertain if a lower temperature is better than 37°C (OR 1.10, 95% CI 0.75 to 1.62; 1 study, N = 412; very low quality-evidence). Multiple pregnancy was reported by two studies, one of which used a paired design, which made it impossible to report the data per temperature. We are uncertain if a temperature lower than 37°C reduces multiple pregnancy (OR 0.80, 95% CI 0.31 to 2.07; 1 study, N = 412; very low-quality evidence). There was insufficient evidence to make a conclusion regarding adverse events, as no studies reported data suitable for analysis.