The Fetal Period

The fetal period follows from the end of the embryonic period, beginning at 9weeks and ending with the onset of labor and birth of the baby. The individual is referred to as a fetus during this period. The period is marked by the continued development and differentiation of structures that emerged during the embryonic period. Basic structures that were laid down in the embryonic period are refined and grow to their final form. Very few new structures appear. Particularly noticeable is the rapid rate of growth during the third and fourth month, with the fetus growing from about 2.5 cm(1 inch) at 8weeks to 13–15 cm (5–6 inches) at 16weeks. It is during this period that the origins of motor, sensory, and learning behavior are to be found.

From the 8th week until birth (around 38 weeks), the developing organism is called a fetus. The fetus is not as sensitive to damage from environmental exposures as the embryo, and toxic exposures often cause physiological abnormalities or minor congenital malformation. All major structures are already formed in the fetus, but they continue to grow and develop.

Weeks 10–12

Gestational age: 9–11 weeks old.

Embryonic age: Weeks nr 8–10. 7–9 weeks old.

• Embryo measures 30–80 mm (1.2–3.2 inches) in length.
• Ventral and dorsal pancreatic buds fuse during the 8th week
• Intestines rotate.
• Facial features continue to develop.
• The eyelids are more developed.
• The external features of the ear begin to take their final shape.
• The head comprises nearly half of the fetus' size.
• The face is well formed
• The eyelids close and will not reopen until about the 28th week.
• Tooth buds, which will form the baby teeth, appear.
• The limbs are long and thin.
• The fetus can make a fist with its fingers.
• Genitals appear well differentiated.
• Red blood cells are produced in the liver.

Fetal Development Week 10

Weeks 13 to 16

Gestational age: 12–15 weeks old.

Embryonic age: Weeks nr 11–14. 10–13 weeks old.

• The fetus reaches a length of about 15 cm (6 inches).
• A fine hair called lanugo develops on the head.
• Fetal skin is almost transparent.
• More muscle tissue and bones have developed, and the bones become harder.
• The fetus makes active movements.
• Sucking motions are made with the mouth.
• Meconium is made in the intestinal tract.
• The liver and pancreas produce fluid secretions.
• From week 13, sex prediction by obstetric ultrasonography is almost 100% accurate.^[11]
• At week 15, main development of external genitalia is finished

Fetal Development Week 13

Week 20

Gestational age: 18 weeks old.

Embryonic age: Week nr 17. 16 weeks old.

• The fetus reaches a length of 20 cm (8 inches).
• Lanugo covers the entire body.
• Eyebrows and eyelashes appear.
• Nails appear on fingers and toes.
• The fetus is more active with increased muscle development.
• "Quickening" usually occurs (the mother and others can feel the fetus moving).
• The fetal heartbeat can be heard with a stethoscope.

Week 23

Gestational age: 22 weeks old.

Embryonic age: Week nr 21. 20 weeks old.

• The fetus reaches a length of 28 cm (11.2 inches).
• The fetus weighs about 925g.
• Eyebrows and eyelashes are well formed.
• All of the eye components are developed.
• The fetus has a hand and startle reflex.
• Footprints and fingerprints continue forming.
• Alveoli (air sacs) are forming in lungs.

Week 27

Gestational age: 26 weeks old.

Embryonic age: Week nr 25. 24 weeks old.

• The fetus reaches a length of 38 cm (15 inches).
• The fetus weighs about 1.2 kg (2 lb 11 oz).
• The brain develops rapidly.
• The nervous system develops enough to control some body functions.
• The eyelids open and close.
• The cochleae are now developed, though the myelin sheaths in neural portion of the auditory system will continue to develop until 18 months after birth.
• The respiratory system, while immature, has developed to the point where gas exchange is possible.

Week 31

Gestational age: 30 weeks old.

Embryonic age: Week nr 29. 28 weeks old.

• The fetus reaches a length of about 38–43 cm (15–17 inches).
• The fetus weighs about 1.5 kg (3 lb 0 oz).
• The amount of body fat rapidly increases.
• Rhythmic breathing movements occur, but lungs are not fully mature.
• Thalamic brain connections, which mediate sensory input, form.
• Bones are fully developed, but are still soft and pliable.
• The fetus begins storing a lot of iron, calcium and phosphorus.
  
  

Week 35

Gestational age: 34 weeks old.

Embryonic age: Week nr 33. 32 weeks old.

• The fetus reaches a length of about 40–48 cm (16–19 inches).
• The fetus weighs about 2.5 to 3 kg (5 lb 12 oz to 6 lb 12 oz).
• Lanugo begins to disappear.
• Body fat increases.
• Fingernails reach the end of the fingertips.
• A baby born at 36 weeks has a high chance of survival, but may require medical interventions.

Weeks 36 to 39

Gestational age: 35–38 weeks old.

Embryonic age: Weeks nr 34–37. 33–36 weeks old.

• The fetus is considered full-term at the end of the 37th week of gestational age.
• It may be 48 to 53 cm (19 to 21 inches) in length.
• The lanugo is gone except on the upper arms and shoulders.
• Fingernails extend beyond fingertips.
• Small breast buds are present on both sexes.
• Head hair is now coarse and thickest.

The development is continued postnatally with adaptation to extrauterine life and child development stages.

Source :

1. http://en.wikipedia.org/wiki/Prenatal_development#Fetal_period

2. http://www.oup.com/

3. http://www.youtube.com

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The embryonic period

The embryonic period begins during the middle of the second week and concludes at the end of the eighth week, at which time the physical appearance of the embryo is clearly human (see Figure 3.1). It is during this time that all the major organs of the body begin to form. It is a time of specialization where cells divide and differentiate to form specific organs, e.g., the heart and lungs.

One of the mysteries of development is how cells ‘know’ to become a heart or lung cell, given that they are all identical at the start of the differentiation process. The local environment of surrounding cells and chemical messages is undoubtedly important, but exactly how one cell becomes a toenail, another a hair, is unknown. During this period the individual is called an embryo. The heart, although only two-chambered, begins to beat and blood is circulated around the embryo by the end of the third week. This enables the removal of waste and the acquisition of nutrients. As all the body’s organs begin to form during this period, it is considered the most critical stage of development.

The embryonic period in humans begins at fertilization (penetration of the egg by the sperm) and continues until the end of the 10th week of gestation (8th week by embryonic age).

The embryo spends the next few days traveling down the Fallopian tube. It starts out as a single cell zygote and then divides several times to form a ball of cells called a morula. Further cellular division is accompanied by the formation of a small cavity between the cells. This stage is called a blastocyst. Up to this point there is no growth in the overall size of the embryo, as it is confined within a glycoprotein shell, known as the zona pellucida. Instead, each division produces successively smaller cells.

The blastocyst reaches the uterus at roughly the fifth day after fertilization. It is here that lysis of the zona pellucida occurs. This process is analogous to zona hatching, a term that refers to the emergence of the blastocyst from the zona pellucida, when incubated in vitro. This allows the trophectoderm cells of the blastocyst to come into contact with, and adhere to, the endometrial cells of the uterus. The trophectoderm will eventually give rise to extra-embryonic structures, such as the placenta and the membranes. The embryo becomes embedded in the endometrium in a process called implantation. In most successful pregnancies, the embryo implants 8 to 10 days after ovulation (Wilcox et al. 1999). The embryo, the extra-embryonic membranes, and the placenta are collectively referred to as a conceptus, or the "products of conception".

Rapid growth occurs and the embryo's main external features begin to take form. This process is called differentiation, which produces the varied cell types (such as blood cells, kidney cells, and nerve cells). A spontaneous abortion, or miscarriage, in the first trimester of pregnancy is usually^[4] due to major genetic mistakes or abnormalities in the developing embryo. During this critical period (most of the first trimester), the developing embryo is also susceptible to toxic exposures, such as:

• Alcohol, certain drugs, and other toxins that cause birth defects, such as Fetal alcohol syndrome
• Infection (such as rubella or cytomegalovirus)
• Radiation from x-rays or radiation therapy
• Nutritional deficiencies such as lack of folate which contributes to spina bifida

Generally, if a structure pre-dates another structure in evolutionary terms, then it often appears earlier than the other in an embryo; this general observation is sometimes summarized by the phrase "ontogeny recapitulates phylogeny." For example, the backbone is a common structure among all vertebrates such as fish, reptiles and mammals, and the backbone also appears as one of the earliest structures laid out in all vertebrate embryos. The cerebrum in humans, which is the most sophisticated part of the brain, develops last. The concept of recapitulation is not absolute, but it is recognized as being partly applicable to development of the human embryo.

Source: http://en.wikipedia.org/, http://www.oup.com, http://www.youtube.com

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