P-551 Association between the length of in vitro culture and mode of ART and the initial se-hCG rise in ongoing singleton pregnancy

Abstract Study question Does the length in vitro and mode of ART (fresh and frozen cycles) impact the se-hCG rise in cycles with ongoing single implantation pregnancy? Summary answer Length in vitro and mode of ART alter the hCG. Blastocysts in FET induced higher hCG than FET clevage stage embryos and fresh blastocyst transfers. What is known already Human chorionic gonadotropin (hCG) is produced by placental trophoblasts providing the first measurable sign of pregnancy. Factors, such as multiple pregnancies, advanced maternal age and gender of the fetus are known to influence hCG levels. Other factors such as mode of ART and length in vitro have been considered to alter the hCG kinetics but with conflicting results. Some studies have shown that hCG levels are higher after fresh blastocyst transfer compared with fresh cleavage stage embryo transfer (ET). Other studies found frozen-thawed embryo transfer (FET) results in higher hCG compared with fresh ET, other studies concluded the opposite. Study design, size, duration Multicenter historical cohort study based on clinical data. The study included 5271 women undergoing in vitro fertilization (IVF), intracytoplasmic sperm injection (ICSI), insemination (IUI) or FET with single ET followed by hCG measurement day 15-19 after ovulation induction (IVF/ICSI/IUI/natural cycle FET) or progesterone administration (estrogen/progesterone substituted FET cycles). All included cycles resulting in single implantation and ongoing pregnancy verified by ultrasound in week 7-8. Data was collected between January 2014 and December 2021. Participants/materials, setting, methods Data was prospectively collected from the Danish Medical Data Centre used by three public fertility clinics, Copenhagen University hospitals. The association between length in vitro (no in vitro period (IUI), day2/3, day5, day6) and mode of ART (IVF/ICSI, FET) and the initial hCG rise was tested both with linear regression and multivariable regression. The results were based on mean serum hCG and presented in percentage with the corresponding 95% confidence interval (CI). Main results and the role of chance After accounting for the pre-defined exclusion criteria (i.e. oocyte donation, pre-implantation genetic testing, more than one gestational sac), 5251 cycles were included in the study. 2122 were FET, 2521 were fresh ET and 608 were IUI. The initial hCG rise was overall lower for fresh ET compared to IUI (no in vitro period). For cleavage stage ET, hCG was 18% (95% CI: 13% - 23%, p < 0.001) lower, and for blastocyst transfer 23% (95% CI: 18% - 28%, p < 0.001) lower. In FET, hCG was 26% (95% CI: 13% - 40%, p < 0.001) higher for blastocyst transfer compared to cleavage stage ET. When comparing blastocyst transfer in FET vs. fresh ET cycles, hCG was 33% (95% CI: 27% - 45%, p < 0.001) higher in FET cycles. Stratifying FET cycles in natural and substituted cycle did not alter the result. All results remained significant after adjusting for referral diagnosis, women age and treatment center. Limitations, reasons for caution It cannot be excluded that the higher level of hCG in IUI pregnancies is due to an additional vanished implantation. Wider implications of the findings The mechanisms in the embryo and endometrial interplay are far from understood. The present data add to the knowledge regarding this, pointing towards alteration in the implantation window in fresh stimulated cycles. Studies following the more detailed hCG rise are needed for further elucidating how ART affect the early implantation. Trial registration number Journal-nr.: 21019857