At four months of pregnancy (around 16 weeks), a fetus is roughly 4.5 to 5 inches long from head to rump and weighs about 3.5 to 4 ounces. That’s comparable in size to an apple or an avocado. While still small enough to fit in the palm of your hand, a 4-month fetus has changed dramatically from just weeks earlier, with recognizable facial features, working kidneys, and the beginnings of coordinated movement.
Length, Weight, and Proportions
The most common way doctors measure a fetus at this stage is crown-to-rump length, meaning from the top of the head to the bottom of the torso (not including the legs). At 16 weeks, that measurement is typically around 4.5 to 5 inches, or roughly 11 to 13 centimeters. If you were to include the legs, total length would be closer to 6 inches.
Weight at this point hovers around 3.5 to 4 ounces (100 to 115 grams), about the weight of a deck of cards. The fetus is gaining weight faster now than in the first trimester, though most of the dramatic weight gain happens in the third trimester. The head still makes up a large proportion of the body, but the rest is catching up quickly as the limbs lengthen and the torso fills out.
Bone and Body Development
By 13 weeks, bones in the skull and long bones of the arms and legs begin to harden. Over the following weeks, this process continues as the skeleton transitions from soft cartilage to firmer bone tissue. The thigh bone (femur) measures about 20 millimeters at the 16-week mark, roughly three-quarters of an inch. Doctors sometimes measure this bone on ultrasound to confirm the fetus is growing on track.
The kidneys are now functional and begin producing urine, which gets released into the amniotic fluid. This is an important step in the development of the urinary system and also contributes to the amniotic fluid that surrounds and cushions the fetus. The digestive system is maturing too, with the intestines settling into their permanent position inside the abdomen after spending earlier weeks partially outside the body cavity.
Skin, Hair, and Outer Features
Between 16 and 20 weeks, a fine layer of hair called lanugo begins to cover the fetus’s body. This isn’t the hair that will remain after birth. Instead, it serves a specific purpose: helping a waxy coating called vernix stick to the skin. Vernix protects the fetus’s delicate skin from constant exposure to amniotic fluid, which would otherwise cause damage over months of immersion. Most of this hair sheds before birth, though some premature babies are still covered in it.
The skin itself is still very thin and somewhat translucent at this stage, with blood vessels visible beneath the surface. The scalp hair pattern is forming, meaning the way hair will eventually grow on the head is already being established. Eyebrows and eyelashes are starting to appear as well, and the facial features are increasingly distinct. The eyes have moved closer to the front of the face, and the ears are near their final position.
Movement and Early Sensory Development
A 4-month fetus is actively moving, flexing its arms and legs, and may even be sucking its thumb. Whether you can feel any of this depends on several factors. Quickening, the term for the first time a pregnant person feels fetal movement, typically happens between 16 and 20 weeks. If you’ve been pregnant before, you may notice it closer to 16 weeks. First-time pregnancies more commonly reach 20 weeks before movement becomes noticeable.
The sensation is subtle at first. Women commonly describe it as fluttering like a butterfly, tiny bubbles popping, light tapping, or small muscle spasms. It’s easy to mistake for gas or digestive activity in the early days. The movements become stronger and more recognizable over the coming weeks as the fetus grows and has less room to move without bumping into the uterine wall.
The nervous system is developing rapidly during this period. While the fetus can’t yet hear or see in any meaningful way, the structural groundwork for those senses is being laid. The inner ear is forming, and the eyes are developing light sensitivity, though the eyelids remain fused shut for several more weeks.
What You See on Ultrasound
If you have an ultrasound around 16 weeks, you’ll likely see a recognizably human shape. The fetus may be moving its arms or legs during the scan, even if you can’t feel it yet. The sonographer will typically measure the head circumference, abdominal circumference, and femur length to assess growth. At this stage, the fetus is large enough that many of these measurements are reliable indicators of whether development is on track.
Some parents learn the sex of the fetus at this point, as the external genitalia are often visible on ultrasound by 16 weeks, though accuracy improves a bit over the next few weeks. The anatomy scan, which is the most detailed ultrasound of pregnancy and checks for structural abnormalities, is usually scheduled between 18 and 22 weeks, just slightly after the 4-month mark.
How Size Varies Between Pregnancies
The measurements above represent averages, and healthy fetuses can fall on either side. The femur length at 16 weeks, for example, ranges from about 17 millimeters at the 5th percentile to nearly 24 millimeters at the 95th percentile. That’s a meaningful spread, and both ends are considered normal. Genetics, the parents’ body size, and the placenta’s efficiency all influence how quickly a fetus grows.
Growth also isn’t perfectly linear. A fetus might measure slightly behind at one appointment and catch up by the next. What matters more than any single measurement is the overall growth trend across multiple checkups. A fetus that consistently tracks along its own curve, even if that curve is on the smaller side, is generally developing well.