Zhang 2018.
| Study characteristics | ||
| Methods | Secondary analysis of the results of a multicentre RCT (Chen 2016)
Conducted: in 14 reproductive medical centres throughout China
Enrolment: June 2013‐May 2014
Power calculation: stated (Chen 2016)
Randomisation: an online central randomisation system (www.medresman.org) was used. Timing of randomisation: after oocyte retrieval Nature of intervention: day‐3 embryo cryopreservation by means of vitrification. Local investigators had the option to transfer day‐2 embryos if there were < 3 embryos on day 2 Follow‐up: until delivery, cLBR, birth outcomes |
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| Participants | 1508 women (746 freeze‐all, 762 control) Inclusion criteria:
Exclusion criteria: history of unilateral oophorectomy, recurrent spontaneous abortion (defined as ≥ 3 previous spontaneous pregnancy losses), congenital or acquired uterine malformations, abnormal results on parental karyotyping, or medical conditions that contraindicated assisted reproductive technology or pregnancy |
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| Interventions | For women who were assigned to the fresh embryo group, on day 3, 2 high‐quality embryos were picked out for fresh transfer and supernumerary embryos were transferred by means of vitrification For women who were assigned to the FET group, there was no fresh transfer as all day‐3 embryos were cryopreserved for later transfer. Local investigators had the option to transfer day‐2 embryos if there were < 3 embryos on day 2. In cycles following the menstrual cycle with ovum pick‐up, on day 4 of the progesterone regimen, 2 day‐3 frozen embryos were thawed and transferred. | |
| Outcomes | Gestational diabetes mellitus, pre‐eclampsia, preterm birth, small for gestational age, and large for gestational age. All outcomes reported for first ET. | |
| Notes | Funding: by the National Basic Research Program of China; the State Key Program of National Natural Science Foundation of China; the National Natural Science Foundation of China; and the Thousand Talents Program | |
| Risk of bias | ||
| Bias | Authors' judgement | Support for judgement |
| Random sequence generation (selection bias) | Low risk | An online central randomisation system (www.medresman.org) was used to automatically generate the assignment sequence. |
| Allocation concealment (selection bias) | Low risk | Assignment sequence was unknown to the clinical investigators. |
| Blinding of participants and personnel (performance bias) All outcomes | Low risk | Blinding of doctors and participants was not possible due to the nature of the intervention. |
| Blinding of outcome assessment (detection bias) All outcomes | Low risk | Outcome assessor blinding was not reported, however primary outcome is not likely to be influenced by lack of blinding. |
| Incomplete outcome data (attrition bias) All outcomes | Low risk | Data were analysed for all randomised women. |
| Selective reporting (reporting bias) | Low risk | All registered outcomes reported. The study used for this follow‐up study was registered in a prospective trials register with the trial number: NCT01841528. |
| Other bias | Unclear risk | (Cumulative) data regarding obstetric complications per subsequent menstrual or cryo‐transfer cycle not reported |
2pn: 2 pro‐nucleate; AMH: anti‐Müllerian hormone; ART: assisted reproductive technology; BMI: body mass index; cLBR: cumulative live birth rate; ET: embryo transfer; FET: frozen embryo transfer; FSH: follicle‐stimulating hormone; GnRH: gonadotropin‐releasing hormone; HBV: hepatitis B virus; hCG: human chorionic gonadotropin; HCV: hepatitis C virus; HSG: hysterosalpingogram; ICSI: intracytoplasmic sperm injection; ITT: intention‐to‐treat; IVF: in vitro fertilisation; LBR: live birth rate; NGS: next‐generation sequencing; OHSS: ovarian hyperstimulation syndrome; PCOS: polycystic ovary syndrome; PGS: pre‐implantation genetic screening; RCT: randomised controlled trial