Embryo Development Stages

The embryonic stage in development during pregnancy is one of the most important, as major body parts are being formed in a short period of time. A woman must take special care during this stage to protect the health of her future baby. Before becoming an embryo, the future fetus originates as an egg that becomes fertilized during ovulation and turns into a ball of cells.

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In women who have a 28-day monthly cycle, ovulation usually occurs about the midpoint, or 14 days before her next period. Each woman's body is different, however, and the timing of ovulation can vary as well. During ovulation, an egg is released and is carried through the fallopian tubes, awaiting fertilization. The cervix softens and produces more mucus, both favorable conditions for sperm to survive and make the journey to the egg. If the egg does not become fertilized during ovulation, the uterine lining sheds and it eventually is discarded through menstruation.


Fertilization happens the moment sperm meets egg. In some cases, two eggs are released, resulting in fraternal twins. If an egg that is released splits into two, identical twins form. After the union of the egg and sperm, the egg travels through the fallopian tube, making its way to the uterus, with the aid of hairlike cilia. At this stage, the egg is now a zygote. The zygote takes about three to five days to make its way to the uterus, where it divides into cells that form into a ball, known as a blastocyst.

Blastocyst Development

Within a week of fertilization, implantation occurs, which is when the blastocyst attaches itself to the uterus. The main cell of the blastocyst will form an embryo, while the rest of the blastocyst's cells form into a placenta. The placenta plays a few important roles. First, it provides nutrients and oxygen to the fetus from the mother, and carries waste from the fetus to the mother. Also, it creates hormones that stop a woman's monthly cycles for the duration of the pregnancy. The amniotic sac, which is where a future fetus will be protected and float in, is formed by cells from the placenta.

Embryo Development

The blastocyst then develops into an embryo on one side of the uterus. An embryo undergoes much development for the next six weeks. Organs form within a week, as the embryo develops from a ball of cells to the form of a human. The spinal cord and brain develop, followed by blood vessels and the heart. The heart even starts to pump fluid through blood vessels. These formations will occur within 20 days of fertilization. If a woman knows she is pregnant at this time, then she needs to take special care to protect her developing embryo, as organ and other developmental malformations occur before 10 weeks into the pregnancy, before the embryo transitions into a fetus.

Fetal Development

An embryo develops into a fetus during the 10th week of pregnancy, or eight weeks after fertilization. During this time, a woman will likely have her first ultrasound, where a technician will check for any abnormalities that may have occurred during the embryo stage.


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