✅ PERIODO FETAL | DESDE LA NOVENA SEMANA AL NACIMIENTO [2023] 📚 EMBRIOLOGÍA HUMANA | RESUMEN

Fetal Period: From the Ninth Week to Birth

Overview of the Fetal Period

• The fetal period marks the development from the ninth week until birth, characterized by significant maturation of tissues and organs alongside rapid body growth.

• This stage is crucial for understanding morphological and functional changes that occur, which are relevant in both embryology and clinical practice.

Key Characteristics of Fetal Development

• The term "fetus" replaces "embryo" during this period; however, susceptibility to teratogenic agents remains, albeit reduced compared to earlier stages.

• Congenital malformations can be identified through various study techniques during this phase.

Measurement Criteria for Fetal Age

• Estimating fetal age relies on two primary criteria: fetal length (measured as cranio-caudal length or vertex-talon length) and weight.

• Weight increases significantly during this period, with a notable gain occurring in the last eight weeks of pregnancy.

Importance of Monitoring Fetal Growth

• Understanding fetal measurements aids in proper pregnancy care, especially for mothers with narrow pelvises or potential developmental abnormalities.

Monthly Development Highlights

Third Month (Ninth to Twelfth Week)

• Morphological Changes:

• The head size decreases relative to body size; facial features become more human-like as eyes move towards their final position.

• Primary ossification centers appear in long bones; external genitalia develop enough for sex determination via ultrasound.

• Functional Developments:

• Endocrine activity begins in thyroid and pituitary glands; kidneys start producing urine by week 12.

Fourth Month (Thirteenth to Sixteenth Week)

• Morphological Changes:

• Head-body ratio decreases; lower limbs grow faster while lanugo hair appears on the head.

• Functional Developments:

• Spleen reaches maximum blood-forming function; swallowing reflex emerges along with initial digestive functions.

Fifth Month (Seventeenth to Twentieth Week)

• Morphological Changes:

• Slower growth observed; vernix caseosa forms along with subcutaneous brown fat contributing to heat production. Hair becomes visible on head and eyebrows.

• Functional Developments:

• Maternal perception of fetal movements begins; heartbeat becomes detectable, and insulin production starts in the pancreas.

Sixth Month (Twenty-first to Twenty-fourth Week)

• Morphological Changes:

• Fetus reaches half its expected length at birth; skin appears wrinkled and translucent allowing visibility of veins. Eyelashes become noticeable.

• Functional Developments:

• Adrenal gland functionality begins, suction reflex is perceptible, and fetus can perceive sounds.

Seventh Month (Twenty-fifth to Twenty-eighth Week)

• Morphological Changes:

Development Stages of the Fetus

Subcutaneous Fat Accumulation and Functional Development

• The lungs and pulmonary vascularization develop sufficiently for adequate gas exchange, while the central nervous system begins to regulate temperature and respiration, albeit imperfectly. Intensive care can support fetal survival if preterm labor occurs during this stage.

Morphological Changes in the Eighth Month

• By the eighth month (29th to 32nd weeks), the skin appears pink, smooth, congestive, and edematous. The trunk and extremities take on rounded contours as lanugo starts to detach. Nails cover their beds but remain soft.

Reflexes and Development in the Ninth Month

• At nine months (33rd to 36th weeks), a pupillary reflex can be induced; if born at this stage, infants typically exhibit a weak cry. Morphologically, hair length increases with isolated lanugo on some trunk areas; navel position rises.

Final Stages of Development

• External genitalia show poor pigmentation with shallow grooves on palms and soles. Skin remains edematous. Functionally, surfactant production supports acceptable respiratory function alongside coordinated suction and swallowing reflexes.

Full-Term Characteristics

• By 37 to 40 weeks, a full-term fetus weighs between 3000g to 3500g and measures 48cm to 52cm in height. The appearance is plump with testicles descended into scrotal bags; areolae pigmentation deepens.

Gestational Duration Calculation

• Pregnancy duration is generally counted from the last menstrual period (LMP), estimated at 40 weeks or 280 days. In embryology, it starts from fertilization lasting about 38 weeks or 266 days.

Due Date Calculation Methods

• Obstetricians calculate due dates using Naegele's rule: subtract three months from LMP date, add one year plus seven days for an estimated delivery date (e.g., LMP September 10 leads to a due date of June 17).

Definitions of Term Length Variations