Once you find out you’re expecting a baby, it’s hard not to wonder exactly what your fetus is doing at any given moment. Other than an ultrasound or two, you don’t get to see your baby developing before they are born. You might wonder how big your fetus is or whether they have fingernails.
We broke down fetus development by week for you. Read on to learn about the amazing changes your baby goes through before birth.
2 Week Zygote
The beginning of pregnancy is about two weeks after your last period, but this is actually right when sperm meets egg. The egg and sperm meet in the ampulla, the second segment of the fallopian tube. The egg contains half of the genetic material your baby will receive and the sperm contains the other half.
3 Week Blastocyst
Your baby is rapidly dividing and multiplying its cells at this stage. It is during that week that identical twins could potentially form, if the zygote splits.
The blastocyst (or blastocysts!) is about the size of a pencil’s point. It has all the genetic material it needs to fully develop into a baby over the next nine months. The blastocyst may implant in the uterus this week or the following week.
4 Week Embryo
This week, your embryo will implant in your uterine lining. It will continue to divide and multiply and its cells will begin to specialize. Your body begins to produce the pregnancy hormone, human chorionic gonadotropin (hCG), after implantation. This means that you can now take a pregnancy test and you might already be able to see a positive result.
5 Week Embryo
Your embryo’s brain and heart have begun to form. Its body is taking shape and it resembles a tadpole. The embryo itself measures about 1/17 of an inch long.
Inside your little tadpole there are now three layers: the ectoderm, the mesoderm, and the endoderm. The ectoderm is the outermost layer and it will become the brain, spinal cord, and the outside of the body. The mesoderm will transform into the bones, muscles, and circulatory system, and the endoderm will become the digestive system and the lungs.
6 Week Embryo
At this stage, your embryo is about one-sixth of an inch long and it has developed a head and trunk. Its heart steadily pumps through the main blood vessels. The backbone and brain begin to form. Limb buds appear—these will become your baby’s arms and legs.
7 Week Embryo
Body systems continue to develop, as do some of the features that make your baby appear a bit more human-like. The umbilical cord is beginning to form. Once the embryo is connected to the placenta via the umbilical cord, it will become a fetus.
One study shows that over 90% of transvaginal ultrasounds done at this time will show a fetal pole with heart motion.
8 Week Fetus
The inner cells of the embryo become the fetus. The outer cells become the amniotic sac, which surrounds the fetus through the duration of the pregnancy. The placenta, which is attached to the uterine wall, begins to grow and nourish the baby.
The fetus is about half an inch long. It is beginning to lengthen and develop smaller body parts such as hands and feet with webbed fingers and toes. It also has the beginnings of eyes, ears, a mouth, and a nose with nostrils.
Within the fetus, lungs and a four-chambered heart have formed, and the brain is growing rapidly. All major organs have begun to form by the end of the eighth week.
Also this week, the baby’s gonads will become either testes or ovaries.
9 Week Fetus
Your baby is now 1 inch long and it has lost its tadpole-like tail. Fingers and toes are just beginning to appear on the hands and feet. The ears and nose are beginning to emerge as well. On the inside, bones and organs are forming and growing.
10 Week Fetus
Your baby is now a little less than 1 1/2 inches long. Tiny toenails and fingernails are growing. External ears are continuing to develop, as well as the upper lip. External genitalia is beginning to differentiate.
The fingers and toes are no longer webbed and now the baby is moving around. You won’t yet feel it, though, because the fetus is still so tiny and is surrounded by amniotic fluid.
11 Week Fetus
Your baby is now 2 inches long, but the head takes up about half of this length. The eyelids have finished forming and are now fused together; baby’s eyes will remain closed for another 12 weeks. The fetus is moving around a lot now: stretching, bouncing, and wiggling.
12 Week Fetus
By week 12, your fetus is approximately 2 1/2 inches long and the brain has begun developing its structures. Your baby has developed sucking reflexes and their kidneys are producing urine.
If your practitioner uses a doppler, there’s an 80% chance you may be able to hear your baby’s heartbeat at this office visit.
13 Week Fetus
Your baby is now 3 inches long. Soft, fine hair called lanugo is starting to grow all over their body. Lanugo helps to protect your baby’s skin while they are in utero. In a few weeks, a waxy substance called vernix will cover your baby’s body to protect its skin from the effects of the amniotic fluid. Lanugo helps hold vernix onto the skin.
Your chance of miscarriage is greatly reduced after 12 weeks gestation.
14 Week Fetus
The fetus is a little over 3 inches long and now has a recognizably human face. It can begin to swallow the amniotic fluid surrounding it. Early sex organs begin to form. At this point during pregnancy, a blood test can tell you your baby’s biological sex.
15 Week Fetus
Your baby is now about 4 1/2 inches long from crown to rump. This measurement is taken from the top of their head to the bottom of their body mass, which is curled up. If they were stretched out to their full length, they would be longer but it’s hard to know exactly how much longer. For this reason, medical professionals generally measure crown to rump.
Your little one is moving around in your uterus. They are covered in a thin layer of skin, through which you can see their developing bones and blood vessels.
16 Week Fetus
Your baby’s ears and eyes are close to reaching their final positions. The nails are well on their way to reaching their full length, which happens at around 17 weeks.
Urine production has increased significantly. The limb movements are becoming more coordinated. At 16 weeks, your baby is about 5 inches long from crown to rump.
You might feel your baby kick for the first time this week, but it’s normal to take longer.
17 Week Fetus
Your baby is now about 5 1/4 inches crown to rump. Fat cells are forming under the skin. Fat works to protect and insulate your little one and it gives them a more baby-like appearance.
This week, your baby will begin sucking and swallowing amniotic fluid. Eventually, sucking and swallowing will become more coordinated and refined to prepare your little one for feeding after they are born.
18 Week Fetus
At 18 weeks, crown-to-rump length is about 6 inches. The fetus weighs about 8 ounces. Your baby’s circadian rhythm is beginning to develop and you might notice more movement at certain times of day. Sex organs are developed enough that an ultrasound tech can fairly reliably predict your baby’s sex at birth.
Your baby can now hear sounds inside your body, such as your heartbeat.
19 Week Fetus
Now your baby measures 6 1/4 inches crown to rump and weighs about 9 1/2 ounces. A white, waxy substances called vernix is beginning to appear on their skin. Vernix helps to protect your baby’s skin from the amniotic fluid surrounding it. It also has other benefits such as protecting from infection and helping your baby regulate body temperature. Most of the vernix will shed before birth, but you might see some when your baby is born.
Your baby is still covered in lanugo, a fine hair over the body, but now they may also have thicker hair on the crown of their head.
20 Week Fetus
You have reached the halfway point of your pregnancy. The fetus’s crown-to-rump length is about 6 3/4 inches, and their weight is about 11 1/2 ounces. A female fetus at 20 weeks already has most of its eggs and a male fetus has developed testes, which are still inside the abdomen at this point.
21 Week Fetus
At 21 weeks, your fetus is now about 7 inches crown to rump and weighs roughly 14 ounces. Now, baby’s digestive system starts gearing up for when it will kick into action, shortly after birth. Your baby is practicing swallowing amniotic fluid, and now the small intestine is beginning to absorb some of these nutrients.
22 Week Fetus
Your baby is over 7 1/2 inches long and weighs over a pound. Their heartbeat can now be heard with a stethoscope and their tear ducts are developing. Their skeleton is hardening and they have more control of hand movement.
23 Week Fetus
At 23 weeks, your fetus is about 8 inches long crown to rump. The baby’s weight is now about 1 pound 4 ounces. Kicking might increase a lot this week, as your little one becomes stronger and more active. Lots of brain development is happening now. Your baby can sense and react to light outside of the womb.
24 Week Fetus
Your baby’s brain is rapidly growing and they are also developing their taste buds. They weigh in at about 1 1/3 pounds and are about 12 inches crown-to-rump length.
They are also beginning to deposit brown fat (adipose tissue) on their body. The purpose of the brown fat is to retain body heat. Newborns may struggle with regulating body temperature at first. This is particularly a problem for a baby born early, since they have not developed enough fat on their body.
A major problem with premature babies is lung development. If preterm labor is detected early, a steroid shot called betamethasone can sometimes be given to enhancing the lung development.
Babies born at this point have some chances of survival with very special care. They will be in the neonatal intensive care unit (NICU), probably for many weeks. In most cases, you can expect them to stay in the NICU until close to their due date.
25 Week Fetus
At 25 weeks, crown-to-rump length is 8 3/4 inches and the fetus weighs about 1 3/4 pounds. Its head and body are covered in lanugo. The lungs have reached the second stage of development, and the blood vessels within them have now formed. Your baby can smell now, and detect odors in your amniotic fluid.
26 Week Fetus
Crown-to-rump length is a little over 9 inches at 26 weeks. The fetus weighs about 2 pounds. Its eyes are now fully formed, with visible eyebrows and eyelashes.
Your baby’s brain is also becoming more complex and has developed a regular sleep-wake cycle. But it might seem as if the baby wants to sleep while you are awake and vice versa!
At 26 weeks, a baby could survive outside the womb with medical intervention.
27 Week Fetus
At 27 weeks, your baby is about 9 1/2 inches from crown to rump and weight about 2 1/4 pounds. There is a lot of brain activity happening at this point. Your baby can now hear sounds outside of your body, such as your voice or your partner’s voice.
Your fetus continues to swallow amniotic fluid and now begins to breathe it in, in practice for breathing air after birth.
28 Week Fetus
28 weeks is the start of your third trimester. Now your fetus measures about 10 inches from crown to rump. It weighs about 2 1/2 pounds.
Eye movement increases around this age, as your baby’s brain continues to develop. A baby delivered at 28 weeks has a good chance of surviving with medical interventions.
29 Week Fetus
A 29-week fetus weighs about 3 pounds and measures about 10 1/2 inches from crown to rump. Its bones continue to harden and the skin thickens. The fine hair covering its body now begins to fall out. You most likely won’t see most of this hair when your baby is born.
30 Week Fetus
Starting at 30 weeks, fetal development is mostly adding fat and muscle. The body is otherwise fully developed at this point. Crown rump length has jumped to about 10 3/4 inches and the fetus weighs roughly 3 1/2 pounds. Hiccups are common at this gestational age.
31 Week Fetus
The fetus measures about 11 inches from crown to rump. It is continuing to gain about a quarter pound each week now, and weighs around 3 3/4 pounds. Your baby is practicing their breathing skills and may blink their eyes.
32 Week Fetus
At 32 weeks, your baby is almost a foot long. They are continuing to gain weight steadily and may be over 4 pounds by now. Your fetus is now able to regulate its own body temperature and it cycles through periods of wakefulness and sleep.
At 32 weeks gestation, many babies will have turned themselves into the head-down position to prepare for birth.
33 Week Fetus
A 33 week fetus is about 12 inches long and weighs about 4 1/2 pounds. The baby is adding fat to its body so that it can stay warm enough after birth.
The skeleton is fully formed and the bones are beginning to harden. Your baby’s eyes have matured enough that they’re able to notice the difference between light and dark.
34 Week Fetus
Your baby is now about 12 1/4 inches long, crown to rump. Their weight has reached about 5 pounds and they continue to put on weight even more steadily now. The fetus begins to shed the vernix that covers their body.
A baby born at 34 weeks has a very good chance of survival with medical intervention.
35 Week Fetus
By 35 weeks, your baby is continuing to grow, reaching about 12 1/2 inches crown to rump and 5 1/2 pounds. Your baby’s physical development is basically complete, but their brain is still growing and changing.
36 Week Fetus
Your baby is now about 13 inches long crown to rump and weighs about 6 pounds. If they have not already turned head down, they may do so around this week. They are not yet full term, but they have a very good chance of surviving if they are born now.
37 Week Fetus
At 37 weeks, your baby is roughly 13 1/4 inches long and now weighs about 6 1/2 pounds. The fetus is practicing sucking in preparation for feeding after birth. They continue to swallow and “breathe” amniotic fluid as well.
38 Week Fetus
If your baby hasn’t been born yet, they are still hanging out in your uterus waiting for that perfect moment. They are now about 7 pounds and almost 14 inches long. The lanugo covering your fetus’s body is mostly gone now, though you might see some of it at birth.
39 Week Fetus
At 39 weeks, your contractions could start at any moment. Your fetus is a fully-formed baby ready, or almost ready, to meet you face to face. The exact length and weight of your little one will vary, but at this point in gestation they are likely around 14 inches crown to rump and 7 1/2 pounds.
