Fetal development in the second trimester

Your baby’s growth accelerates during the second trimester. You can feel their movements more acutely. Their senses are beginning to develop. Your belly is also growing. By the end of the second trimester, your baby will measure about 25 cm from crown to rump and weigh around 1 kg. Here are the main stages of development, week by week.

To learn more about the earlier and later stages, read our fact sheets on fetal development during the first and third trimesters.

This fact sheet focuses on the baby’s development. To learn more about how mothers experience the second trimester, you can read our fact sheet entitled Second trimester: Physical changes and common discomforts.

Week 15: Baby is beginning to move

Your baby begins a period of accelerated growth 13 weeks after conception. They measure about 9.5 cm from crown to rump and weigh around 80 g.

Their network of blood vessels is becoming more complex. To support their small, developing body, their heart works steadily, pumping about 24 L of blood per day. Their heart rate is between 140 and 150 beats per minute.

Your baby is increasingly mobile in the womb. For example, they can now turn their head, which measures about 3.5 cm. Since they still have a lot of space in the uterus, and their movements are quite subtle, it’s normal that you can’t feel them yet. Don’t worry, those kicks and punches are coming!

Your baby’s taste buds are now functional, which means you can introduce them to your favourite foods. Flavours in your diet are passed on to them through your amniotic fluid.

Week 16: Baby is stronger and more agile

Your baby is stronger and their kicks are more pronounced.

Their movements are also better coordinated. When they touch the umbilical cord, their first reaction is to move away. But soon they’ll grip, pull, and push the cord as if it’s a toy. Their little hands will become more agile.

Under your baby’s eyelids, their eyes are beginning to move. Because your baby is drinking a lot of amniotic fluid (400 ml per day), they often get the hiccups. This is perfectly normal. In addition, their digestive system is starting to produce a blackish substance called meconium, which accumulates in their intestines.

At the end of this week, your baby will measure about 11.5 cm from crown to rump and weigh around 110 g.

Week 17: Baby is growing hair

Inside your baby’s body, fat is beginning to form. This is very important. The fat (known as brown fat) will help your baby stay warm after birth, since newborns don’t yet have the reflex to shiver. This reserve will also give them energy in their first moments of life. Your baby is starting to develop antibodies.

By week 17, all their organs and systems have formed. However, several stages of development are still to come. Their skeleton is still growing. For now, it’s mostly soft cartilage. Your baby measures about 13 cm from crown to rump and weighs around 150 g.

Fine downy hair is slowly appearing.

Week 18: Is baby a boy or a girl?

At this stage, your baby’s proportions change: their head is now smaller than the rest of their body.

Blood flow reaches their extremities, which means they’re moving a lot more! In the uterus, they’re turning over, wiggling their arms and legs, and touching their toes and face. You may feel very light movements, which feel like flutters or small waves.

Additionally, a fatty protective layer (myelin) now coats their nerve fibres, like a plastic sheath around an electrical wire. Myelin is what conducts nerve impulses, or information exchanged between your baby’s central nervous system (brain and spinal cord) and peripheral nervous system.

Soon, their central nervous system cells—namely, their brain cells—will be enveloped in myelin. Once that happens, connections can be made and neural networks can be consolidated.

Your baby’s retinas are sensitive to light. If a strong light is projected onto your belly, they’ll turn away and cover their eyes.

An ultrasound is performed at 18–20 weeks. The main purpose of this exam is to ensure that your baby’s organs are developing properly (e.g., heart, kidneys, central nervous system). The sex can also be identified. If you’re having a boy, his penis is now visible. If you’re having a girl, her vulva and uterus are visible.

By the end of week 18 (or week 16 since conception), your baby measures about 14 cm from crown to rump and weighs around 200 g.

Week 19: Baby is developing all five senses

This week, your baby is going through an important change: the areas of their brain dedicated to the five senses are forming. That means they’re starting to process sensory information. A whole new world is opening up to them!

Neuron production is slowing. Soon, neural connections will be created and consolidated. This is how learning begins.

This week, their skin is developing a white, waxy coating known as the vernix caseosa. It serves to protect your baby’s delicate skin from friction and irritation.

They’re growing steadily but they still have enough room in the uterus to move their arms and legs. They measure around 15 cm from crown to rump and weigh about 260 g.

Week 20: Baby’s movements are more noticeable

In the womb, your baby is learning to move their body parts. Their muscles are bigger and stronger, and their movements are more vigorous. At this stage, most moms start to feel their baby. Between active periods, your baby is sleeping 18–20 hours a day.

Their skin, which is still rather transparent, is beginning to thicken. The first folds of their cerebral cortex are developing.

By the end of week 20 (or week 18 after conception), your baby measures about 16 cm from crown to rump and weighs around 320 g.

Week 21: Baby is playing with their hands and feet

Your baby’s body is fully formed. By now, they measure about 18 cm from crown to rump and weigh around 390 g. During the next month, they’ll be gaining a lot of weight.

They’re becoming more and more familiar with touch. They play catch with their feet and sometimes suck their thumb. They also like playing with the umbilical cord.

Their lungs are continuing to mature. Your baby is learning to expand their chest muscles, which will help them breathe after birth.

Their eyeballs are formed but they don’t have colour (pigment) in their irises. Their lips are becoming fuller and they have more hair.

Week 22: Baby can taste sugar

Your baby already likes the taste of sugar! The sweeter the amniotic fluid, the more they drink. But thanks to their pancreas, which is now secreting insulin, their blood sugar level is controlled.

As your baby grows, their fat reserves remain limited. Their skin is still wrinkled but will stretch as they gain weight.

During this second half of pregnancy, your baby’s brain is developing at high speed. In their gums, two sets of teeth are forming: baby teeth and, right behind them, permanent adult teeth. The baby teeth will be ready to come out in the months following birth.

At this stage, your baby measures about 19 cm from crown to rump and weighs around 460 g.

Week 23: Baby is opening their eyes

Your baby’s body is now covered in vernix caseosa. This white, waxy coating is very important, as the amniotic fluid has an increasingly high concentration of urine. As you approach your due date, this protective layer will gradually fade. The thin coating that remains will facilitate delivery.

By the beginning of your sixth month, your baby’s muscle development is in full swing. You may notice that they’re particularly active at certain times. This is a sign that they’re doing well! Try to keep track of their movements.

Your baby is also starting to open their eyes! Their eyelids are coming apart gradually. Within a few weeks, they’ll be looking around. They’re becoming aware of light and darkness.

If you’re having a boy, his testes are slowly descending from his belly to his future scrotum. If you’re having a girl, her reproductive system is fully formed. Her ovaries include all the eggs she will ever have.

Your baby now measures around 20 cm from crown to rump and weighs about 540 g.

Week 24: Baby is growing real hair

Your baby’s immune system is producing white blood cells, which will allow their body to fight off disease and infection.

Their skin is thickening and has a light pigment, but is still quite transparent. Their blood vessels (capillaries) are clearly visible.

Real hair has started to replace your baby’s fine downy hair. Their fingernails are more distinct.

During this period, weight gain is very important. Your baby now weighs over 630 g and measure about 21 cm from crown to rump. They’re taking up more space in the uterus.

If they were born today, 22 weeks after conception, your baby could survive but would need highly specialized care. At this stage, they would be very premature, as their respiratory and digestive systems are not fully developed.

Week 25: Baby knows you’re there!

Your baby is beginning to recognize certain elements or situations: your voice and the voices of other family members, daylight, certain activities, the silence of the night, and more. They’ll sometimes jolt inside your belly if they’re surprised, and may respond to belly touches. In short, they’re aware of your presence.

They also react to the sounds your body makes. They can hear your heartbeat, the air entering your lungs, and the gurgle of your digestive system. All of these sounds calm them and lull them to sleep. It’s no surprise that, when moms hold their baby after delivery, they quickly calm down. They’re reassured by all the familiar sounds they heard while in the womb.

By now, you can feel your baby’s movements. When they’re awake, they’re quite active. They may move 20–60 times every half hour.

Their bronchial tubes are almost fully formed, but their lungs haven’t reached maturity.

Their eyes gradually take on pigmentation, although this colour will change after birth. The ivory that protects the enamel of their baby teeth begins to form.

Depending on your baby’s position, a person could hear their heartbeat by pressing their ear against your belly. Your little one measures about 22.5 cm from crown to rump and weighs around 720 g.

Week 26: Baby’s facial features are becoming clearer

This week, your baby’s lungs are developing. This is a particularly important stage for premature babies. The lungs are beginning to secrete a substance that will allow them to expand more easily. This substance also protects them from infection, increasing your baby’s chances of survival outside the womb.

Their facial features are taking shape: their eyelashes are formed, their nose is clearly defined, their hairline is sometimes visible, and their neck is more pronounced. From now until birth, your baby’s face won’t change very much! They’re opening their eyes more frequently.

Their nerves are almost fully developed, and their brain now has more convolutions (folds in the cerebral cortex). These convolutions are also deeper. Synapse development will accelerate until week 32. Synapses are connections between brain cells that allow the exchange of information. Of all your baby’s systems, the nervous system develops slowest. It will continue to mature until the end of pregnancy.

Since the beginning of the second trimester, your baby has doubled in size. They measure about 23 cm from crown to rump. They’re also a lot heavier, weighing around 820 g. They’re a third of the weight they’ll be at birth. Already, your baby has less room to move in utero.

Week 27: Baby might be dreaming!

This week, your baby has acquired an essential reflex that will help them communicate their needs once they’re born: crying. Their survival depends on it!

Their circadian rhythm is more regular, but that doesn’t mean that it’s perfectly aligned with yours. Are they dreaming? Some experts say that at this stage of development, babies are already having dreams. Fast, low-amplitude brain waves—meaning beta or REM waves—have been detected. These are associated with REM sleep, when you usually dream.

Your baby’s little hands are active. They’re working on their dexterity! Occasionally, they suck their thumb. It calms them down and allows them to strengthen their jaw and cheek muscles.

Blood vessels now supply their lungs, which are not yet mature. Their bones are also stronger. If they were born this week, your baby would still need extensive medical care to ensure their survival. They would be very premature.

They measure about 24 cm from crown to rump and weigh around 920 g.

Week 28: Baby is becoming curious

Your baby is rounding out and looking very cute! Their skin is smoother and slightly redder due to many small blood vessels. They’ve built up a good layer of fat under their skin. This week, it accounts for approximately 3% of their total body weight.

Gradually, the placenta is becoming less important as a source of nutrients. In utero, the level of amniotic fluid is decreasing. Your baby is taking up much more space. Their swallowing and sucking reflexes are also improving.

They’re curious: when they sense light or hear a sound, they turn their head in that direction.

Their liver and spleen are working hard to produce a large quantity of blood cells.

Your baby is continuing to gain weight. They measure about 25 cm from crown to rump and weigh almost 1 kg.

• Fetal development: 3rd trimester
• Fetal and embryo development: The first trimester

Scientific review: Dr. Françoise Rypens, radiologist, CHU Sainte-Justine, and Dr. Chantal Lapierre, head of medical imagery, CHU Sainte-Justine Research and copywriting:The Naître et grandir team Updated: April 2021

Photo: 123rf/denisnata