The 17th week of pregnancy marks a significant point in the second trimester, characterized by rapid fetal growth and development. This stage is often called the “honeymoon phase” because many early, uncomfortable symptoms have subsided, allowing expectant parents to focus on the tangible progression of the pregnancy. Tracking the fetus’s growth via size comparisons and physical measurements helps visualize the remarkable transformation occurring within the uterus. Understanding these weekly changes provides a scientific context for the physical experience of mid-pregnancy.
The 17-Week Size Comparison
At 17 weeks, the fetus is approximately the size of a turnip or a large pear. The average length, measured from the crown to the rump, is about 5.1 to 5.5 inches (13 to 14 centimeters). This measurement method is transitioning because the legs are now more developed and extended, which will soon allow for a head-to-heel measurement.
The fetus weighs roughly 4.9 to 6.3 ounces (140 to 180 grams), having nearly doubled its weight in the previous two weeks. This rapid gain signals the start of a phase where the body prepares for life outside the womb. The placenta may now be smaller than the fetus itself, illustrating the speed of the current growth trajectory. The body’s proportions are also becoming more human-like, with the head now more proportional to the rest of the body.
Fetal Development Milestones
The internal activity this week focuses on maturation and insulation, preparing the fetus for temperature regulation after birth. A primary development is the initial formation of brown fat, a specialized type of adipose tissue. This fat layer is deposited under the skin and serves the purpose of generating heat, as newborns cannot shiver to keep warm.
The skeleton is transforming, with soft cartilage beginning to harden in a process called ossification. This progressive bone hardening provides structure and strength for the growing body. The umbilical cord is growing longer and thicker to accommodate the increased nutritional demands of the developing fetus.
Sensory development continues as the ears have reached their final position, and the sense of hearing is maturing. Loud noises outside the uterus may cause the fetus to startle, demonstrating a functional auditory system. Sweat glands are beginning to develop beneath the skin, although they will not become fully active until later. The external genitalia are now fully formed, making the sex of the fetus often discernible during a routine ultrasound.
Maternal Changes During Week 17
The growth of the fetus and supporting structures begins to have noticeable effects on the expectant parent’s body. The uterus is growing upward and outward, rising higher in the abdomen and often transitioning the physical appearance to a more obvious pregnancy shape. This change in uterine position may alleviate the frequent urination experienced in the first trimester, as pressure on the bladder is temporarily reduced.
Quickening, the first perception of fetal movement, may be felt around this time, especially in second or subsequent pregnancies. These initial movements are usually felt as light flutters, bubbling, or gas-like sensations in the lower abdomen. The body’s increased blood volume, which supports the growing placenta and fetus, can lead to other physical changes.
This increased blood flow is responsible for the “pregnancy glow” that some people experience, but it can also contribute to symptoms like nasal congestion and dizziness. The stretching of the round ligaments, which support the expanding uterus, can cause occasional sharp, quick pains in the lower belly or groin area. While the second trimester generally brings increased energy, the body works hard to sustain this rapid growth.