(fē'təs)
n., pl., -tus·es.
The unborn young of a viviparous vertebrate having a basic structural resemblance to the adult animal.
In humans, the unborn young from the end of the eighth week after conception to the moment of birth, as distinguished from the earlier embryo.
Alternate Names : Zygote, Blastocyst, Embryo, Fetus
Definition
The progressive growth that occurs between fertilization of an egg to the birth of a baby.
Description
Pre-embryonic stage
The pre-embryonic stage starts with fertilization and lasts through the first two weeks of pregnancy.
FERTILIZATION. During intercourse, the male ejaculates and releases semen into the woman's vagina. The semen contains 50 to 200 million spermatozoa per milliliter that reach the cervix within 90 seconds and the outer end of the fallopian tubes within five minutes. Fertilization results when a single sperm, or spermatozoon, penetrates an ovum. The chromosomal material of the ovum and spermatozoon then combine, forming a zygote.
IMPLANTATION. After fertilization, the zygote travels three or four days through the fallopian tube toward the body of the uterus. During this time, the cell starts to divide. By the time the zygote reaches the body of the uterus, it consists of 16 to 50 cells and is called a morula. The morula collects large cells at the periphery of the ball and becomes an outer casing with a connected inner group of cells surrounded by a fluid space. At this stage, the structure is termed a blastocyst. The blastocyst implants on the inner layer of the uterus, called the endometrium, approximately eight to 10 days after fertilization, where it will obtain nourishment. In as many as 50% of all pregnancies, the zygote fails to reach the implantation stage, in which it becomes an embryo.
Embryonic stage
The embryonic stage spans the third week to the eighth week of pregnancy.
Body organs form out of layers of tissue called germ cells. The three distinct layers of germ cells are called the endoderm, the ectoderm, and the mesoderm. Newborns with a congenital defect originating from one of the three layers should be evaluated for malformations that developed
from the same germ cells. For example, the heart and kidneys form from the mesoderm. A baby born with a heart defect should also have an x ray or ultrasound of the kidneys.
During the embryonic period, the baby is most susceptible to invasion by teratogens, which are substances that can result in birth defects. Women should be instructed on ways to reduce their contact with teratogens.
Information:
When sperm is deposited in the vagina, it travels through the cervix and into the Fallopian tubes.
Conception - interactive tool
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 zygote 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 baby. The blastocyst sticks tightly to the lining, where it receives nourishment via the mother's bloodstream.
Cell division
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
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