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Fetal development

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Illustrations

Fetus (10 weeks old)
Fetus (10 weeks old)
Fetus (12 weeks old)
Fetus (12 weeks old)
16-week fetus
16-week fetus
24-week fetus
24-week fetus
26 to 28-week fetus
26 to 28-week fetus
30 to 32 week fetus
30 to 32 week fetus
3.5 week fetus
3.5 week fetus
Fetus (7.5 weeks old)
Fetus (7.5 weeks old)
8.5 week fetus
8.5 week fetus

Alternative Names    Return to top

Zygote; Blastocyst; Embryo; Fetus

Information    Return to top

When sperm is deposited in the vagina, it travels through the cervix and into the Fallopian tubes.

ZYGOTE

A single sperm penetrates the mother's egg cell, and the resulting cell is called a zygote. The zygote contains all of the genetic information (DNA) necessary to become a child. Half of the genetic information comes from the mother’s egg and half from the father’s sperm. The zygote spends the next few days traveling down the Fallopian tube and divides to form a ball of cells.

BLASTOCYST

The zygot continues to divide, creating an inner group of cells with an outer shell. This stage is called a blastocyst. The inner group of cells will become the embryo, while the outer group of cells will become the membranes that nourish and protect it.

The blastocyst reaches the womb (uterus) around day 5, and implants into the uterine wall on about day 6. At this point in the mother's menstrual cycle, the lining of the uterus has grown and is ready to support a fetus. The blastocyst sticks tightly to the lining, where it receives nourishment via the mother's bloodstream.

EMBRYO

The cells of the embryo now multiply and begin to take on specific functions. This process is called differentiation. It leads to the various cell types that make up a human being (such as blood cells, kidney cells, and nerve cells).

There is rapid growth, and the baby's main external features begin to take form. It is during this critical period (most of the first trimester) that the growing baby is most susceptible to damage. The following can interfere with the baby's development:

WEEK BY WEEK CHANGES

The following list describes specific changes that occur in the womb:

The end of the eighth week marks the end of the "embryonic period" and the beginning of the "fetal period".

Update Date: 10/19/2007

Updated by: Deirdre O’Reilly, MD, MPH, Neonatologist, Division of Newborn Medicine, Children’s Hospital Boston and Instructor in Pediatrics, Harvard Medical School, Boston, Massachusetts. Review Provided by VeriMed Healthcare Network.

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