The beginning of human personhood is the moment when a human is first recognized as a person. There are differences of opinion about the precise time when human personhood begins and the nature of that status. The issue arises in a number of fields, including science, religion, philosophy, and law, and is most acute in debates about abortion, stem cell research, reproductive rights, and fetal rights.

Traditionally, the concept of personhood has included the concept of the soul, a metaphysical concept of a non-corporeal or extra-corporeal dimension of human beings. In modernity, the concepts of subjectivity and intersubjectivity, personhood, mind, and self have come to encompass a number of aspects of humanness that were previously considered to be characteristics of the soul. One question about the beginning of human personhood has been the moment at which soul enters the body. An alternative question, both historically and in modern times, may be at what point does the developing individual acquire personhood or selfhood.

Issues relating to the question of the beginning of human personhood include the legal status, bodily integrity, and subjectivity of mothers, and the philosophical concept of natality, i.e. "the distinctively human capacity to initiate a new beginning" that a new human life embodies.

Discussions of the beginning of personhood may be framed in terms of the moment life begins. James McGrath and others argue the beginning of personhood begins is not interchangeable with the beginning of a human life. According to Jed Rubenfeld, the terms human being and person are not necessarily synonymous.

Embryologist Scott Gilbert states:

There is no consensus among biologists as to when personhood begins. Different biologists have proposed that personhood begins at such events as fertilization, gastrulation, the acquisition of an EEG pattern, and birth. Other scientists claim that the acquisition of personhood is gradual or that the question of personhood is not a biological one.

Fertilization is the fusing of the gametes; a sperm cell and an ovum (egg cell) fuse to form a single-cell zygote. This is the beginning of the diploid phase of the human life cycle after two genetically unique haploid cells created via meiosis and chromosomal translocation combine their DNA and begin to develop into a multi-cellular organism. The zygote is genetically distinct from each of its parents.

The process of fertilization lasts around 24 hours. Many zygotes die shortly after fertilization, most often due to chromosomal abnormalities. Estimates of the percentage that die prior to implantation vary from 10% to 70%. Those that fail to implant in the uterine wall are sloughed off with the endometrial lining during menstruation. Of those that implant, many are miscarried, often without the woman knowing she was pregnant; estimates of post-implantation loss also vary considerably. As cleavage (cellular division) occurs, a single zygote may split into two or three zygotes, resulting in monozygotic ("identical") twins or triplets. At other times, two individually fertilized zygotes may fuse into one zygote, which is known as a chimera.