A woman cannot naturally become pregnant without sperm; this would be a case of parthenogenesis, which fails in mammals because of genomic imprinting.
What is parthenogenesis?
Parthenogenesis is a form of asexual reproduction in which an embryo develops from an unfertilized egg. The term comes from the Greek words for "without father."
Why it works in some animals but not in mammals
Many reptiles, fish, amphibians, and birds can produce offspring through parthenogenesis. In these species the egg’s genome alone is sufficient to support full development.
Mammals, however, rely on a strict balance of maternal and paternal gene expression. This balance is set by genomic imprinting, where certain genes are turned on or off depending on whether they come from the mother or the father.
The role of genomic imprinting
During gamete formation, DNA methylation marks certain genes. In mammals, paternally expressed genes drive placental growth, while maternally expressed genes regulate fetal growth. An egg lacking paternal marks cannot produce a functional placenta, so the embryo stops developing early.
Laboratory attempts in mice
Scientists have produced a few healthy mice from unfertilized eggs by manipulating imprinting marks and fusing two maternal genomes. The process is inefficient and requires many steps, including epigenetic editing and surrogate gestation.
Human context and medical implications
In humans, spontaneous activation of an unfertilized egg can lead to ovarian teratomas or partial hydatidiform moles, but never a viable pregnancy. Research focuses on creating parthenogenetic embryonic stem cells for regenerative medicine, not on human reproduction.
Ethical and practical limits
Human reproductive parthenogenesis is considered impossible and ethically unacceptable. The scientific community does not pursue it, focusing instead on safe stem‑cell applications.