Fetal Development Stages: A Comprehensive Guide

Fetal Development Stages

Overview

Fetal development primarily involves rapid body growth, tissue differentiation, and the formation of organs and systems. The fetal period is marked by significant growth, especially in the final weeks.

Viability

Fetal viability refers to the ability of a fetus to survive outside the uterus (after premature birth). Fetuses weighing less than 500g at birth typically do not survive. With postnatal care, some fetuses born weighing less than 500g may survive. Many term pregnancies with low birth weight result from intrauterine growth retardation (IUGR). The majority of fetuses born weighing between 1500g and 2500g survive but may experience complications. Prematurity is a common cause of morbidity and perinatal death.

Clinical Pregnancy

Clinical pregnancy is divided into three trimesters:

1. First Trimester: Formation of key systems.
2. Second Trimester: The fetus may survive if born prematurely.
3. Third Trimester: The fetus matures and reaches major developmental milestones. By 35 weeks of gestation, a weight of approximately 2500g indicates fetal maturity.

During the second and third trimesters, various structures can be identified and measured using ultrasound. Key measurements for estimating fetal age include:

1. Biparietal diameter (BPD): Diameter of the head between the two parietal bones.
2. Head circumference
3. Waist circumference
4. Femur length
5. Foot length

Detailed Fetal Development by Week

9-10 Weeks

At 9 weeks after fertilization (10 weeks after the last menstrual period [LMP]), the fetus is 4 inches long. Development during this period focuses on the growth and maturation of structures formed during the embryonic period.

• Broad face
• Wide-set eyes
• Low-set ears
• Fused eyelids
• Short legs and relatively small thighs
• Similar male and female external genitalia
• Visible bowel loops in the proximal umbilical cord until mid-10th week; return to the abdomen by the 11th week

12 Weeks

• Uterus palpable above the symphysis pubis
• Fetus size: 6-7 cm, 13g
• Ossification centers appear
• Finger differentiation
• Nose and skin development
• Fine hair (lanugo) begins to develop
• External genitalia begin to differentiate
• Spontaneous movements begin
• Rapid growth between the 13th and 16th weeks

14 Weeks

• Coordinated limb movements (starting at the end of the embryonic period - 8 weeks) become more refined but remain subtle
• Limb movements visible on ultrasound
• Slow eye movements begin
• External genitalia recognizable
• Smaller head compared to 12 weeks
• Lengthened lower limbs
• Bones clearly visible on ultrasound due to active ossification

16 Weeks

• Scalp hair pattern determined
• Ovaries differentiated and containing primordial follicles with oogonia
• Fetus has human-like features, with eyes positioned anteriorly
• Size: 10 cm, weight: 110g
• Sex determination possible (from 14 weeks)
• Almost all organs formed
• Eyes still closed, but hands and feet begin to move

20 Weeks

• Size: 22 cm
• Weight: 300g
• Less transparent skin
• Lanugo covers the body
• Hair begins to form