Obstetrics and Gynecology - A. M. Hromova 2000

Fertilization and Fetal Development
Development of the Zygote
Dynamics of Fetal Development

At the end of the first month (4 weeks) of development, the curved embryo is 1–1.5 cm in length. In appearance, it is indistinguishable from animal embryos. By the end of the 2nd month (8 weeks), the fetus reaches 2–2.5 cm in length; its HEAD is not overly large relative to the torso, and the limbs are distinctly differentiated. By the end of the 3rd month (12 weeks) of Pregnancy, the fetus is 9–10 cm long and weighs 20–25 g. It acquires a human-like appearance, and the external genitalia begin to differentiate. By the end of the 4th month (16 weeks), the fetus measures 15–17 cm, weighs 120 g, and its sex is clearly identifiable. Lanugo appears on the body surface. At 5 months (20 weeks), the fetal length is 25–26 cm and the weight is 280 g. The head remains disproportionately large and is covered with Hair. Subcutaneous fat begins to accumulate. A fetus born alive at this stage is capable of superficial respiratory movements, but it is not yet viable, and Signs of Life fade quickly. By the end of the 6th month (24 weeks), the fetal length is 28–30 cm, and the weight is 680 g. Subcutaneous fat increases, and lanugo covers the entire body. The Skin is coated with vernix caseosa. A fetus born alive during this period breathes and moves its limbs, but soon dies. At 7 months (28 weeks), the fetus measures 35–38 cm and weighs 750–1000 g. The skin is thin and red, with scant subcutaneous adipose tissue. A baby born at this gestational age moves, breathes, and emits a weak cry; it is now viable. At 8 months (32 weeks), the fetus is 40–42 cm long and weighs 1500 g or more. The skin is abundantly covered with lanugo, and subcutaneous fat is poorly developed, giving the skin a pinkish hue. The pupillary membrane is absent, and the face appears wrinkled. At 9 months (36 weeks), the fetus reaches 45–46 cm in length and weighs about 2500 g. The subcutaneous fat layer is well developed. In appearance, the fetus closely resembles a full-term infant. Newborns at this age breathe well, cry loudly, nurse effectively, and generally survive. At 10 months (40 weeks), the fetal length reaches 50–52 cm, and the weight is 3000–3500 g. The newborn is considered mature and fully viable.

There is a direct correlation between fetal length and gestational age. According to F.N. Haase's formula, up to five months of pregnancy, the fetal length in centimeters equals the square of the gestational age in obstetric months. After five months of pregnancy, the length equals the number of obstetric months multiplied by 5. Thus, ontogeny—which serves as a abbreviated repetition of phylogeny—is a complex, genetically determined mechanism influenced by numerous internal and external environmental factors. It culminates in the birth of a mature human Organism capable of extrauterine survival.