The 18th week of pregnancy marks a period of notable growth and refinement within the second trimester. At this stage, the fetus is transitioning from rapid structural formation to increasing size and maturing organ systems. The visual appearance becomes distinctly human, with features becoming more defined and movements more noticeable.
Current Size and Growth Milestones
The fetus now measures approximately 5.5 inches in length and weighs about 7 ounces. The baby is comparable in size to a large sweet potato or a large bell pepper. This measurement is taken from the crown of the head to the rump because the legs remain curled up against the body.
Medical professionals use the crown-to-rump measurement because accurately measuring the full length to the heel is difficult at this stage. This method is standard for assessing growth during the early second trimester. Soon, as the body lengthens and straightens, the measurement will shift to the crown-to-heel method.
Developing Facial and Limb Features
By 18 weeks, the face has a much more recognizable human profile, moving past the earlier developmental stages. The eyes have migrated from the sides of the head to their final position facing forward. Similarly, the ears have moved into their proper place on the sides of the head and are beginning to stand out slightly from the skull.
The fetus engages in a range of visible facial movements, including stretching, yawning, and forming distinct expressions. These actions are supported by the maturing nervous and muscular systems. On the hands and feet, the intricate patterns that form the unique fingerprints and toeprints are beginning to develop.
Limb movements are becoming more coordinated and frequent, involving actions like grasping and opening and closing the hands. These movements, such as kicking and twisting, are often observable during ultrasound examinations. The active motion within the amniotic fluid contributes to the strengthening of muscles and joints, preparing them for a life outside the womb.
Skin and Protective Coverings
The skin of the fetus remains relatively thin, making the small blood vessels beneath the surface slightly visible. Two specialized layers are forming on the exterior to provide protection in the fluid-filled environment. One layer is a fine, downy hair called lanugo that begins to cover the body.
Lanugo typically starts to develop around the 16th to 20th week of gestation. This hair helps anchor the second specialized covering, a greasy, white substance known as vernix caseosa. Vernix is a thick, protective coating composed of shed skin cells and secretions.
The function of the vernix caseosa is to shield the delicate skin from constant exposure to the amniotic fluid. This coating acts as a barrier, preventing the skin from becoming overly saturated and protecting it from minor irritants. Together, the lanugo and vernix ensure the skin remains healthy and intact.