Fetal Development

A close look at the stages of prenatal development

Fetal development, prenatal development or antenatal development is the process during which an embryo gestates inside the uterus. It encompasses the time from fertilization to birth. Embryology is the branch of biology that deals with studying this process.

Typically, the process is broken down in a chronological manner, with focus placed on the development of major organs and tissues on a week-by-week basis. Though there is minor disagreement among experts regarding exact timelines, most consider there to be two distinct stages of fetal development in humans: the embryonic period and the fetal period.

Stages of Fetal Development

The embryonic period is generally defined as the time from the moment of fertilization to the end of the eighth week of gestation. During these 56 days, the embryo grows from the size of a pin head to nearly an inch in length. It forms the foundations of the features that make it uniquely human, including bones, internal organs, a brain and nervous system, and even fingers and toes. It's also a time when the fetus is very vulnerable to toxins and other environmental factors that can cause abnormalities and even lead to miscarriage.

The fetal period is the time between the ninth week of gestation and the birth of the baby, which usually takes place about 30 weeks later. The embryo, now considered to be a fetus, embarks on a phase of dramatic growth. The tissues and organs that developed during the embryonic period mature at a rapid rate. Bones harden, body fat increases and the fetus develops external genitalia (making it possible to determine its sex using ultrasound technology).

The Fetal Life-Support System

It's truly amazing that a cluster of cells can grow from a tiny speck into a full-grown, healthy baby over the course of nine months, but it wouldn't be possible without the support of a remarkable system of supporting organs. The placenta, umbilical cord and amniotic fluid each perform a crucial task that helps ensure the fetus gets the nutrition and protection it needs to develop. Understanding the unique roles that each of these organs perform during pregnancy can help you gain an appreciation for exactly what's happening inside the womb during pregnancy.