You're finally here. The second trimester. Honestly, the first three months usually feel like a marathon run through a fog of nausea and bone-deep exhaustion, so hitting the four-month mark feels like a massive win. This is the "sweet spot." But as that tiny bump starts to actually look like a bump and not just a heavy lunch, the big question is always: how big is it really? People talk about fruit sizes all the time, but the size of fetus at 4 months is about much more than just a piece of produce. It is a period of explosive, almost violent growth.
By the time you hit week 13 to 16—which is roughly where the fourth month sits—your baby is no longer just a cluster of cells or a tadpole-looking embryo. It’s a human. A tiny, translucent, incredibly active human.
The "Fruit" Comparison and Why It’s Kinda Misleading
We’ve all seen the apps. One week it’s a lemon, the next it’s an avocado. By the end of the fourth month, specifically around week 16, your fetus is roughly the size of a large avocado or a pomegranate. We're talking about 4.5 to 5 inches from crown to rump.
But here’s the thing—measuring a baby isn't like measuring a box. Because the legs are often curled up, doctors usually measure "CRL" or Crown-Rump Length. If you were to stretch those little legs out, the total length would be even more impressive. At 4 months, the weight is somewhere around 3.5 to 5 ounces. It’s light. It's like holding a couple of letters in your hand. Yet, that small weight is enough to start making your lower back ache and your jeans feel like a torture device.
The growth is non-linear. One week they might put on a few grams, and the next, they’ve doubled their weight. It’s a literal growth spurt happening inside your uterus. This is why you might feel fine on Tuesday and feel like your skin is stretching to its limit by Thursday.
Beyond Length: Proportions Are Shifting
In the first trimester, the head is huge. Like, comically large compared to the body. It’s a "Megamind" situation because the brain needs so much real estate to develop. But as you enter the fourth month, the body starts to catch up. The neck lengthens. The chin finally lifts off the chest.
Everything is becoming more symmetrical.
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The ears are moving from the neck up to the sides of the head. The eyes, which started on the sides like a bird’s, are migrating toward the front of the face. It’s during this phase that the baby starts looking like your baby.
Bones and Skin
At 14 weeks, the skin is still incredibly thin. It’s basically see-through. If you could look inside, you’d see the entire network of blood vessels and the skeleton, which is still mostly flexible cartilage. But by week 15 and 16, ossification—the process of turning that cartilage into hard bone—is kicking into high gear. This happens first in the long bones of the arms and legs and in the skull.
Interestingly, the skin also starts getting covered in lanugo. This is a fine, downy hair. It’s weird, right? A furry baby? But it’s there for a reason. Since there isn't much body fat yet to keep them warm, this hair helps the vernix (that waxy white coating) stick to the skin to protect it from the amniotic fluid. Without it, the baby’s skin would get totally pruned up from being submerged in liquid for nine months.
Movement You Can (Maybe) Feel
This is the big one. This is the month where "Quickening" happens.
Quickening is that first moment you feel the baby move. Because the size of fetus at 4 months is now substantial enough to actually make contact with the uterine walls, you might start feeling flutters.
- Some people describe it as bubbles popping.
- Others say it feels like a tiny goldfish swimming around.
- A few people swear it just feels like gas.
If this is your first pregnancy, you might not recognize it until week 20 or even later. But if you’ve been pregnant before, you know what you’re looking for, and you might catch those movements as early as week 16. The baby is actually incredibly active—flipping, kicking, and even sucking its thumb—but because they are still so small and cushioned by a literal bag of water, most of those "gymnastics" go unnoticed by the parent.
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The Sensory Explosion
While the baby is growing in inches, they are also growing in "humanity." This is the month where the nervous system really starts to wire up.
The fetus can now make facial expressions. They aren't "happy" or "sad" in the way we think, but the muscles are firing. They squint. They frown. They grimace. They are practicing for their big debut.
More importantly, they can start to hear. The tiny bones in the middle ear are hardening. They can hear the "whoosh" of your blood, the gurgle of your stomach, and the muffled vibration of your voice. Research from institutions like the Mayo Clinic suggests that by the end of this month, the fetus may even react to external sounds or bright lights directed at the mother’s abdomen.
Misconceptions About the 4-Month Bump
A common myth is that your bump size directly correlates to the size of fetus at 4 months. It doesn't.
I’ve talked to many people who were stressed because they weren't "showing" enough at 16 weeks. Or conversely, they felt huge. Your bump is influenced by:
- Abdominal tone: Stronger abs hold the uterus in longer.
- Height: Longer torsos give the baby more vertical room to hide.
- Pregnancy count: If this is your second or third, your muscles are already "relaxed," and you’ll pop much earlier.
- Bloat: Progesterone slows down digestion. A lot of that 4-month "baby" bump is actually just digestive backup.
The baby itself is only the size of an avocado, remember? The rest of the "size" is the placenta, the amniotic fluid, and your own shifting organs. Your uterus is roughly the size of a large cantaloupe right now, and it’s pushing your intestines up and out of the way.
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Developmental Milestones at 16 Weeks
| Feature | Development Status |
|---|---|
| Genitals | Usually visible on ultrasound, though not 100% guaranteed. |
| Scalp | Hair patterns are starting to form. |
| Heart | Pumping about 25 quarts of blood per day. |
| Lungs | Practicing "breathing" by inhaling amniotic fluid. |
The heart is a powerhouse. At this stage, your doctor can usually find the heartbeat easily with a Doppler. It sounds like a galloping horse, usually ranging between 120 and 160 beats per minute. That’s about twice as fast as your own heart.
The Mystery of the Fingerprints
By the end of the fourth month, something incredible happens: the fingertips have developed their unique ridges. The baby now has their own permanent, individual fingerprints. It’s a wild thought that while they are still small enough to fit in the palm of your hand, they already have the one thing that will identify them for the rest of their lives.
What You Should Be Doing Now
Since the baby is going through a massive skeletal building phase, your body’s demand for calcium is peaking. If you aren't getting enough, the baby will literally pull calcium from your own bones. It’s a bit parasitic, but that’s the reality of fetal development.
- Boost your Calcium and Vitamin D: Think yogurt, sardines (if you can stomach them), or fortified plant milks.
- Side Sleeping: Your uterus is getting heavy enough now that it can start to compress the vena cava (a major vein) when you lie on your back. Start practicing sleeping on your side.
- Hydration: The volume of amniotic fluid is increasing rapidly. You need water to keep that reservoir full and to support your increased blood volume.
Navigating the "Anatomy Scan" Anxiety
Most people get a big ultrasound between weeks 18 and 22, but some get an early look at month four. It’s normal to feel anxious. You’re thinking about the size, the heart, the limbs. Just remember that at 4 months, the baby is still in a "building" phase. If you have an ultrasound now, the baby might look a bit "scary" or skeletal. That’s just because they haven't put on their "baby fat" yet. That comes in the third trimester. Right now, they are all about length and structural integrity.
Actionable Next Steps
Monitor your energy levels. The "honeymoon phase" of the second trimester is the best time to start any nursery projects or travel, as your mobility is still high but the morning sickness has likely faded.
- Schedule your 20-week anatomy scan if you haven't already; these slots fill up fast.
- Invest in a pregnancy pillow. Even though the fetus is small, your center of gravity is shifting.
- Moisturize your belly. The skin is beginning its most rapid expansion phase right now.
- Talk to your baby. Their hearing is online. It sounds cheesy, but it starts the bonding process and helps them recognize your voice after birth.
The size of fetus at 4 months is a bridge between the microscopic beginnings and the robust infant you’ll meet in a few months. It’s a time of quiet but intense transformation. Enjoy the flutters—they’ll be full-blown kicks soon enough.