Maternal hCG concentrations in early IVF pregnancies: associations with number of cells in the Day 2 embryo and oocytes retrieved.

STUDY QUESTION Do number of cells in the transferred cleavage stage embryo and number of oocytes retrieved for IVF influence maternal hCG concentrations in early pregnancies? SUMMARY ANSWER Compared with transfer of a 2-cell embryo, transfer of a 4-cell embryo results in higher hCG concentrations on Day 12 after transfer, and more than 20 oocytes retrieved were associated with low hCG concentrations. WHAT IS KNOWN ALREADY Maternal hCG concentration in very early pregnancy varies considerably among women, but is likely to be an indicator of time since implantation of the embryo into the endometrium, in addition to number and function of trophoblast cells. STUDY DESIGN, SIZE, DURATION We followed 1047 pregnancies after IVF/ICSI from oocyte retrieval until Day 12 after embryo transfer. Women were recruited in Norway during the years 2005-2013. PARTICIPANTS/MATERIALS, SETTING, METHODS Successful pregnancies after transfer of one single embryo that had been cultured for 2 days were included. Maternal hCG was quantified on Day 12 after embryo transfer by chemiluminescence immunoassay, which measures intact hCG and the free β-hCG chain. Information on a successful pregnancy, defined as birth after >16 weeks, was obtained by linkage to the Medical Birth Registry of Norway. MAIN RESULTS AND THE ROLE OF CHANCE Transfer of a 4-cell embryo resulted in higher maternal hCG concentrations compared with transfer of a 2-cell embryo (134.8 versus 87.8 IU/l, P < 0.05). A high number of oocytes retrieved (>20) was associated with low hCG concentrations (P < 0.05). LIMITATIONS, REASONS FOR CAUTION The factors studied explain a limited part of the total variation of hCG concentrations in early pregnancy. Although embryo transfer was performed at the same time after fertilization, we do not know the exact time of implantation. A further limitation to our study is that the number of pregnancies after transfer of a 2-cell embryo was small (27 cases). WIDER IMPLICATIONS OF THE FINDINGS Number of cells in the transferred embryo and number of oocytes retrieved may influence the conditions and timing for embryo implantation in different ways and thereby influence maternal hCG concentrations. Such knowledge may be important for interpretation of hCG concentrations in early pregnancy.