For some it dates from the first instant; for others it could not at least precede nidation implantation in the uterus.    From Wordnik.com.  [Do Embryos Have Souls?] Reference

Pregnancies are medically recognized conceptions, often defined as beginning at implantation or nidation roughly a week after fertilization.    From Wordnik.com.  [The Population of the United States] Reference

Adverse effects on nidation in rats, embryo viability in mice, and metabolism in vitro of rat testis and human spermatozoa have been attributed to Lithium, as have teratogenicity in submammalian species and cleft palate in mice.    From Wordnik.com.  [TreeHugger] Reference