At 16 weeks pregnant, your baby is about 7 inches long and weighs roughly 5 ounces, or about the size of an avocado. That’s measured from head to heel. This is a big leap from just a few weeks earlier, and your baby is now large enough that you might start feeling the first flutters of movement.
Size and Proportions at 16 Weeks
Seven inches and 5 ounces gives you a general picture, but every baby grows at a slightly different pace. NICHD growth charts show that normal head circumference at 16 weeks ranges from about 112 to 135 millimeters (roughly 4.4 to 5.3 inches), and abdominal circumference falls between about 91 and 116 millimeters. That’s a wide spread between the 5th and 95th percentiles, which means two perfectly healthy 16-week babies can differ noticeably in size on an ultrasound.
Your baby’s head is still proportionally large compared to the body, but the rest is catching up. The legs are now longer than the arms, and limb movements are becoming coordinated enough to show up clearly on ultrasound.
What Your Baby Can Do Now
At 16 weeks, your baby’s kidneys are actively producing urine, which has become a major contributor to the volume of amniotic fluid surrounding them. The bladder fills and empties on a cycle of roughly every 55 to 155 minutes. This urinary system activity is one reason your provider monitors fluid levels at ultrasound appointments.
The heart is pumping hard. A normal fetal heart rate at this stage sits between 110 and 160 beats per minute, which is roughly twice as fast as yours. You won’t feel it, but your provider can pick it up with a handheld Doppler device at your prenatal visit.
Limb movements are becoming more coordinated. Your baby can flex, extend, and move arms and legs in patterns that look less like random twitches and more like purposeful motion. These movements are building the muscle and nerve connections your baby will need after birth.
Skin, Hair, and Protective Layers
Between 16 and 20 weeks, a layer of ultra-fine hair called lanugo begins to cover your baby’s body. This soft fuzz works alongside a waxy coating called vernix to protect delicate skin from the amniotic fluid. Without that protection, weeks of constant fluid exposure could irritate and damage the skin. The lanugo will mostly shed before birth, though some premature babies are born with traces of it still visible.
At this point, your baby’s skin is still thin enough to be somewhat translucent. Blood vessels are visible beneath the surface. Over the coming weeks, layers of fat will fill in underneath, giving the skin a more opaque appearance.
The 16-Week Ultrasound
If you have an ultrasound around 16 weeks, one of the things your technician can attempt is determining your baby’s sex. The external genitalia are fully formed by now, so identification is generally accurate when the baby cooperates. The catch is positioning. If your baby is curled up or facing the wrong direction, the technician may not get a clear view, and you might need confirmation at a later scan.
Beyond sex determination, this ultrasound gives your provider a chance to measure your baby’s head, abdomen, and femur length. These measurements get plotted on growth charts to make sure development is tracking within the expected range. A single measurement that falls slightly outside the average isn’t usually a concern on its own. Your provider looks at the overall pattern across multiple visits.
Feeling Your Baby Move
Sixteen weeks is right at the early edge of when you might start to feel your baby’s movements, a sensation called quickening. If you’ve been pregnant before, your body is more tuned to recognize those subtle flutters, and you may notice them around 16 weeks. First-time mothers typically don’t feel movement until closer to 20 weeks. Early fetal movement often feels like bubbles, light tapping, or a gentle rolling sensation that’s easy to mistake for gas.
Don’t worry if you feel nothing at 16 weeks. Your baby is moving, as the ultrasound will confirm, but at 5 ounces, those kicks and stretches simply aren’t strong enough for many women to detect yet. The placenta’s position matters too. A placenta attached to the front wall of the uterus (anterior placenta) acts as a cushion, muffling sensations and often delaying quickening by a few extra weeks.
What Changes in the Weeks Ahead
Growth accelerates from here. Over the next four weeks, your baby will roughly double in weight. The skeleton, currently made mostly of soft cartilage, will begin hardening into bone. The nervous system is rapidly wiring itself, and by around 18 weeks, your baby will start responding to sound as the hearing structures mature. Fat deposits will begin forming under the skin, and your baby’s features will become increasingly defined on ultrasound images.
For you, the second trimester is often when nausea fades and energy returns. Your uterus is now about the size of a small melon, sitting roughly halfway between your pubic bone and navel. As your baby grows from avocado-sized toward the length of a banana over the next few weeks, you’ll likely notice your belly becoming visibly rounder.