Word parts
zyg/o joined, yoked-ote a thing that is
What it means
The first cell of a new human, made when the sperm and egg unite, usually in the fallopian tube. It has a full set of chromosomes, half from each parent. Within hours it starts dividing as it travels to the uterus, becoming a ball of cells that implants in the uterine wall about a week after fertilization; from then on, the developing baby is called an embryo, and later a fetus.
Related terms
fetus
An unborn baby from about the ninth week of development until birth.
fertilization
The joining of a sperm and an egg to form a single new cell, the start of a pregnancy.
first trimester
The first part of pregnancy, from the first day of the last menstrual period through the end of week 13.