For the MS protocol, thawing kits were equilibrated overnight in a 37°C incubator. Warming procedures utilized the kits (Cryotech RtU, Japan). To remove the cryoprotectants, blastocysts were warmed, and cryoprotectants were diluted in a three-step process.

The warming process starts with the exposure of blastocysts to thaw solution (TS) with 1M trehalose for one minute at 37°C. Subsequently, the blastocyst will be transferred to a second well containing a dilution solution (DS) of 0.5M trehalose for a two-minute rinse at room temperature. This is followed by two additional three-minute and 30-second rinses in the wash solution (WS) at room temperature. The timeline for standard warming of blastocysts requires a total of 6.5 min. After thawing, embryo will be placed in the incubator at least 2 hours before transfer.

Serum ß-hCG ≥25mIU/mL

The implantation rate is explained as the number of gestational sacs per number of embryos transferred

diagnosed by ultrasonographic visualization of one or more gestational sacs or definitive clinical signs of pregnancy at 6 weeks or more after the onset of the last menstrual period. In addition to intra-uterine pregnancy, it includes a clinically documented ectopic pregnancy.

A pregnancy outside the uterine cavity, diagnosed by ultrasound, surgical visualisation, or histopathology

Having at least one gestational sac on ultrasound at 12 weeks' gestation with heart beat activity

The spontaneous loss of an intra-uterine pregnancy before 22 completed weeks of gestational age

Multiple definitions, defined as delivery at <24, <28, <32, <37 completed weeks

Structural or functional disorders that occur during intra-uterine life and can be identified prenatally, at birth or later in life. Congenital anomalies can be caused by single gene defects, chromosomal disorders, multifactorial inheritance, environmental teratogens and micronutrient deficiencies. The time of identification should be reported. Any congenital anomaly will be included as followed definition of congenital abnormalities in Surveillance of Congenital Anomalies by Division of Birth Defects and Developmental Disabilities, NCBDDD, Centers for Disease Control and Prevention (2020).

Weight of singletons and twins

Weight < 2500 gm at birth

Weight < 1500 gm at birth

Weight over than 4.500 g for women with diabetes, and a threshold of 5.000 g for women without diabetes

The admittance of the newborn to NICU

≥2 gestational sac at early pregnancy ultrasound

Birth of more than one baby beyond 22 weeks

The death of a fetus prior to the complete expulsion or extraction from its mother after 20 completed weeks of gestational age. The death is determined by the fact that, after such separation, the fetus does not breathe or show any other evidence of life, such as heartbeat, umbilical cord pulsation, or definite movement of voluntary muscles.

Death of a live-born baby within 28 days of birth. This can be divided into early neonatal mortality, if death occurs in the first seven days after birth, and late neonatal if death occurs between eight and 28 days after delivery

Total direct cost to have a live birth after embryo transfer. Direct cost include medical consultations, ovulation stimulation drugs, laboratory and embryology services, ultrasound scanning, medical procedures such as oocyte retrieval and embryo transfer, hospital charges, nursing and counselling services and administrative and overhead charges. Cost data will be collected for a supplementary analysis and will be reported in a separated paper.