An embryo is an early stage development of a multicellular organism. Organs and crucial body structures are typically formed at this stage of human development. Before about the 10th day of gestation, the embryo is referred to as a blastocyst. The embryo is referred to as the fetus after the 11th week of pregnancy.

What is an embryo?

An embryo is a diploid organism in the early stages of development during pregnancy. If it develops properly, it will become a fetus.

An embryo can be created naturally (through sexual intercourse) or through in vitro fertilization. After a sperm fertilizes an egg, the combination, known as a zygote, moves down the fallopian tube over the course of several days. It begins dividing into an inner and outer group of cells called a blastocyst. Once an amniotic sac develops — typically around day 10–12 of gestation — it’s known as an embryo.

The embryo continues to develop and grow while attached to the uterine wall, forming main systems including the brain, spinal cord, heart, and external features. After approximately eight weeks, the embryo has become a fetus, which can continue to develop until birth.

Embryos and male fertility

While “embryo” is technically the term for 10–12 day fertilized eggs, it’s often used in fertility medicine to refer to blastocysts created in the lab. Embryo quality refers to the genetic and physical health of fertilized eggs created using in vitro fertilization.

Male health and fertility may play a role in embryo development even after a sperm has successfully fertilized an egg. Research shows that higher sperm motility (movement) was associated with the development of higher quality blastocysts, while older age had a negative effect. Sperm DNA fragmentation may also affect embryo quality, with some research indicating that DNA fragmentation in sperm may impair embryo development. More research is needed in this area.

Embryo vs. fetus

Fetus is the developmental stage between embryo and birth. The transition from an embryo to a fetus occurs around the eighth week of pregnancy, after the embryo has started forming its major organs, body structures, and sex organs, and has a regular heartbeat. The new fetus is only around 1 inch long, but has more human features. The fetus continues to develop and grow, forming a complete skeleton, skin, lungs, hair, and other structures until birth.