Human development begins at fertilization when an oocyte (ovum) from a female is fertilized by a sperm (spermatozoon) from a male. Development involves many changes that transform a single-cell, the zygote, into a multicellular human being.
Embryology is concerned with the origin and development of a human being from a zygote to birth.
Before We Are Born. Essentials of Embryology and Birth Defects. Keith L. Moore, T.V.N. Persaud, Mark G. Torchia. 9th Edition. Copyright 2016 by Saunders, an imprint of Elsevier, Inc.