embryo
Meaning of embryo: Weeks counted from fertilization and weeks counted from a last period differ by two, which is why embryo timelines often seem to disagree.
Definition of embryo
An embryo is the earliest developmental stage of an animal or plant after fertilization, the period in which the basic body plan and the foundations of the organs are laid down. In humans it runs from roughly the second week after fertilization to the end of the eighth, after which the developing organism is called a fetus.
What happens during the embryonic stage?
The first fortnight after fertilization is spent dividing, travelling down the tube and implanting in the wall of the uterus, with no recognizable body yet. The embryonic stage proper begins when the cell mass folds itself into three layers, each of which will build a defined set of structures: the outer layer forms skin and the nervous system, the middle layer forms muscle, bone, heart and kidneys, and the inner layer forms the lining of the gut and airways. Within these weeks the neural tube closes, the heart begins to beat, limb buds appear and lengthen, and the face takes shape. By the close of the stage every major organ system exists in rough draft.
How does an embryo differ from a fetus?
| Embryo | Fetus | |
|---|---|---|
| Timing after fertilization | Weeks two to eight | Week nine until birth |
| What is happening | Structures are being formed for the first time | Structures already present grow and mature |
| Sensitivity to exposures | Highest, because a forming organ can be permanently misshapen | Lower for structure, though growth and the brain remain vulnerable |
| Appearance | Curved, with a tail-like end and paddle-shaped limb buds | Recognizably human in proportion |
The boundary is defined by the completion of organ formation rather than by a size or a single event, which is why the transition from embryonic to fetal development is conventional rather than sharp.
Why do the dates seem to disagree?
Two clocks run at once. Developmental biology counts from fertilization. Clinical pregnancy dating counts from the first day of the last menstrual period, about two weeks earlier, because that date is usually known while the moment of fertilization is not. The end of the eighth embryonic week therefore falls around the tenth week of pregnancy on a clinical chart. The same offset explains why the period of greatest sensitivity to infections, medicines and alcohol is often described as the first trimester: it covers weeks when many people do not yet know a pregnancy exists.
Used in a sentence
Ultrasound at this stage of pregnancy was used to assess embryo development and confirm cardiac activity.
Good to know
The change from embryo to fetus is marked by the completion of major organ formation rather than by a fixed size, and is placed by convention at the end of the eighth week after fertilization, which falls near the tenth week when a pregnancy is dated from the last menstrual period.
Word origin
Where “embryo” comes from
From Greek embryon, meaning that which grows within, formed from en-, meaning in, and bryein, meaning to swell or grow.