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Fetal Development in the First Trimester

Fetal Development in the First Trimester During pregnancy, the fetus undergoes three major stages of development: pre-embryonic period (fertilization to week 3); embryonic period (weeks 4 through 7); fetal period (week 8 through birth).

Stages of fetal development. The First 4 Weeks

The pre-embryonic phase starts with ovum fertilization and last 3 weeks. As the zygote passes through the fallopian tube, it undergoes a series of mitotic divisions, or cleavage.

As the fertilized ovum advances through the fallopian tube toward the uterus, it undergoes mitotic division, forming daughter cells. Each contains the same number of chromosomes as the parent cell. The first cell division ends within 1 week pregnancy approximately 30 hours after fertilization. At 1 week pregnant the zygote develops into a small mass of cells called a morula, which reaches the uterus at about 3rd day after fertilization. At the beginning of 2 weeks pregnancy the zygote becomes blastocytes and it is attached in the endometrium and when the 3 weeks pregnancy sets in, the blastocytes sinks below the endometrium surface is. The penetration site seals and restores the continuity of the endometrial surface and finally before the 4 weeks pregnancy ends the embryonic period begins.

The Next Month

During the embryonic period, the developing zygote starts to take on a human shape and is now called an embryo. Each germ layer eventually forms specific tissues in the embryo. The three germ layers are:

  • Ectoderm;
  • Mesoderm;
  • Endoderm.

At 5 weeks pregnancy the ectoderm as the outermost layer develops into the epidermis, nervous system, pituitary gland and tooth enamel. The mesoderm as the middle layer forms the connective and supporting tissue. The endoderm as the innermost layer forms the pharynx and trachea. At 6 weeks pregnancy the development of salivary glands, optic lens, blood and vascular system, musculature, auditory canal and alimentary canal starts. At 7 weeks pregnancy lining of lower portion of anal canal, the mesothelial lining of heart, pleural, and peritoneal cavities including the liver, pancreas and bladder is forming. Finally at 8 weeks pregnancy the embryo begins to develop its hair, kidney and ureters, bladder and urethra and prostate for male.

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In other words, the organ systems form during the embryonic period. During this time, the embryo is particularly vulnerable to injury by maternal drug use, certain maternal infections, and other factors.

The Rest Of The Way

During the fetal stage of development, which extends from eight week until birth, the maturing fetus enlarges and grows heavier.

From Embryo To Fetus

First Month Of Pregnancy

At the end of the first month, the embryo has a definite form. The head, the trunk, and the tiny buds that will become the arms and legs are discernible. The cardiovascular system has begun to function, and the umbilical cord is visible in its most primitive form.

Second Month Of Pregnancy

During the second month, the embryo – called a fetus before 9 weeks pregnancy begins may grow to 1” (2.5 cm) and weighs 1/30 oz (1g). The head and facial features develop as the eyes, ears, nose, lips, tongue, and tooth buds form. The arms and legs also take shape. Although the gender of the fetus isn’t yet discernible, all external genetalia are present. Cardiovascular function is complete, and the umbilical cord has a definite form. At the end of the second month, the fetus resembles a full-term neonate except for size.

Third Month Of Pregnancy

During 10 weeks pregnancy up to the end of 13 weeks pregnancy, the fetus grows to 3” (7.6 cm) and weighs 1 oz (28.3g). Teeth and bones begin to appear, and the kidney starts to function. The fetus opens its mouth to swallow, grasps with its fully developed hands, and prepares for breathing by inhaling and exhaling (although its lungs aren’t functioning). At the end of the first trimester (the 3-month periods into which pregnancy is divided), its gender is distinguishable.

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The fetus’ head is disproportionately large compared to its body. (This features changes after birth as the infant grows.) The fetus lacks subcutaneous fat. (Fat starts to accumulate shortly after birth.)

Structures unique to the fetus include fetal membranes, the umbilical cord, the placenta, and amniotic fluid. During placentation, chorionic villi invade the decidua and become the fetal portion of the future placenta. The embryo at 4 weeks as a normal fetus begins to shoe noticeable signs of growth.

The first 3 months of fetal development is mainly focused with the organ development. This could simply mean that any unhealthy activities such as smoking and drinking alcohol even medicines are strictly prohibited. The main reason for this is to prevent the risk of having congenital problems. It is also suggested that week by week baby development is very crucial at this period. It is best for the mother to get rid of stress and anxiety that can cause emotional problems. Researchers made a medical investigation about the impact of the emotional aspect can influence the development fetus.

Delores C. West is a compassionate healthcare professional with a focus on women's health and wellness. As a certified nurse-midwife, Delores is dedicated to providing comprehensive care to women throughout their reproductive journey. With a warm and nurturing approach, she empowers her patients to make informed decisions about their health and well-being. Delores's expertise in women's health makes her a trusted resource for individuals seeking personalized and compassionate care. Connect with her on LinkedIn to learn more about her commitment to women's health and wellness.