Definition: Given an abstract, generate a keyword (a noun phrase) that best describes the focus or contribution of the paper. Such keywords can be directly from the given abstract or outside it.
Input: Abstract: The human chorionic gonadotropin (hCG) beta subunit curve of normal singleton in vitro fertilization (IVF) pregnancies shows cyclic peaks during the first 56 days after embryo transfer (ET). This phenomenon is not demonstrable in normal spontaneous conceptions. The hCG curves of 151 singleton IVF pregnancies achieved by transfer of two, three, and four embryos therefore were compared statistically with the hCG curves of singleton IVF pregnancies achieved by transfer of only one embryo. The hCG levels of singleton IVF pregnancies achieved by transfer of more than one embryo had statistically higher hCG levels at 7 and 8, 19 and 20, and 37 and 38 days after ET. In a separate evaluation it was observed that declines in hCG levels of normal singleton IVF pregnancies preceded four of five first-trimester abortion incidence peaks of singleton IVF pregnancies. The observed cyclic hCG secretion may be indicative of early embryo deaths. This observation supports the contention that multiple ET results in significantly more multiple implantations than previously appreciated. Thus embryo survival in utero may represent another major contributory factor to success or failure of IVF.

Periodicity