Fetal development week by week
Follow your baby's development from a tiny mass of cells through to a fully developed baby. Our illustrations show how your baby is growing inside your uterus, while our Inside pregnancy videos take a 3D animated look at a baby from conception to labour and birth. And see the whole nine months in one timeline!
Fetal development - conception
Are you ready? Many women, if they're trying to become pregnant, decide to stop drinking, smoking and taking drugs – even over-the-counter ones – before they conceive so that their body will be in the best possible shape for baby-making.
Strange as it may seem, your doctor or midwife will calculate your due date (and your baby's gestational age) by working back to the first day of your last menstrual period. Since it's often difficult to know exactly when you're ovulating, and consequently when sperm fertilised your egg, medical experts often use your last menstrual period as the starting point for the next nine months. This means the first 'official' week of your pregnancy is actually the week you started your last period. As a result, your baby's real age is two weeks behind his gestational age.
During the second week – as it does each month in anticipation of new life – oestrogen triggers your uterus (womb) to form a lush, blood-rich lining of tissue. Rising levels of the hormone progesterone prepare your uterus to support a fertilised egg, and to house your growing baby over the coming months. At the same time, the ovaries ripen eggs in fluid-filled sacs called follicles. During this brief period of your body's preparation, conception can result once a sperm meets the egg.
By the third week, things really start to happen. Around mid-cycle (day 14 of a typical 28-day cycle), you ovulate – one of your eggs is swept into your fallopian tube. In the next 12 to 24 hours, that egg can be fertilised if one of 350 million sperm (an average ejaculation) manages to swim all the way to the fallopian tube – from the vagina through the uterus – to penetrate the egg.
Once fertilised, the egg, now called a zygote, immediately closes its outer membrane to the rest of the 250 (on average) sperm that have made it this far. Then it begins dividing into identical cells as it is swept down the fallopian tube to the uterus, where it will continue to grow. You probably won't know you're pregnant for a few weeks yet, though some women maintain they know the exact moment they conceived.
Once you are pregnant, you may begin to worry about a host of things: what foods are safe, is it fine to have a hot bath, is it safe to use a computer screen? Check out our Is it safe to...? section for some reassuring answers.
Note: Every baby develops differently. This page is designed to give a general idea of how a fetus grows in the uterus.
Fetal development - 4 weeks pregnant
The ball of cells – now called an embryo – growing inside your uterus (womb) is the size of a poppy seed. It's dividing into three layers that will later form organs and tissues.
The neural tube – from which the brain, backbone and spinal cord, and nerves will sprout – develops in the top layer. The heart and the circulatory system begin to appear in the middle layer. The third layer starts to house the lungs, intestines and beginnings of the urinary system.
The yolk sac produces red blood cells and nutrients for your baby. But early versions of the placenta and umbilical cord are already at work, and preparing to take over this job.
The placenta's cells are burrowing into the lining of your uterus, creating microscopic fingers of tissue (chorionic villi). Once the placenta is fully functioning by the end of this week, it will be able to make nutrients for your baby and take away his waste products. The umbilical cord delivers those nutrients, and oxygen, to your baby.
If you haven't already tried a home pregnancy test, taking one now will confirm you are pregnant. There are other signs that show you could be pregnant. Check our due date calculator to work out when your baby is due. Once you get confirmation, you can call your GP and make an appointment. Good and consistent antenatal care is one of the best ways to ensure that you and your baby will be healthy throughout pregnancy.
Note: Every baby develops differently. This page is designed to give a general idea of how a fetus grows in the uterus.
Fetal development - 5 weeks pregnant
You won't look pregnant at this point but your baby’s tiny heart is already dividing into chambers and will begin pumping blood soon. Your baby is about a quarter of a centimetre long and looks more like a tadpole than a human being. He'll have a growth spurt this week - the first of many!
He's busily growing all his major organs, including the kidneys and liver. His intestines are developing and the appendix is in place. The neural tube, which connects his brain and spinal cord, will close this week.
All these internal developments are matched by changes on the outside, too. The buds that will grow into your baby’s arms and legs begin to sprout. And below the opening that will later form your baby's mouth, small folds exist where the neck and the lower jaw eventually develop.
Fetal development - 6 weeks pregnant
By the time you're six weeks pregnant, your baby is about the size of a lentil. Her heart has started beating at about 150 beats per minute. That's about twice the rate of yours. If you could see inside yourself, you'd find she has an oversized head in proportion to her body. Your baby’s facial features are forming, with dark spots where the eyes are, openings where the nostrils will be, and pits to mark the ears.
Protruding buds that will become her arms and legs are even more noticeable now. Muscle and bone tissues are already building up. And her pituitary gland, which releases hormones, is forming, along with the rest of her brain.
While this amazing transformation is taking place in your uterus, you may notice a few changes in yourself. Feeling moody one minute and joyful the next? What you're experiencing is normal. It's partly down to your fluctuating hormones. But it's hardly surprising if you feel a bit emotional, considering how your life is all set to change. Take a look at our early pregnancy resource centre, which will guide you through the first few weeks of pregnancy. You can also find out more about being six weeks pregnant.
Fetal development - 7 weeks pregnant
Now about one half of an inch / 1.25 centimetres long, the size of a chick pea. The embryo has distinct, slightly webbed fingers and toes. It's a jumping bean, moving in fits and starts. The liver is churning out large amounts of red blood cells until the bone marrow forms and takes over this role.
The eighth week marks the beginning of a very busy developmental stage. Between now and 20 weeks, your baby will be growing rapidly, and body parts that formed in the first few weeks of life (such as the heart and brain) will become more specialized and complicated. Right now, the teeth and palate are forming, while the ears continue to develop. The fetus' skin is paper thin, and veins are clearly visible.
You're in the throes of the first trimester, when many women complain of common pregnancy aches and pains. Keeping food down may be next to impossible, thanks to morning sickness, caused in part by the pregnancy hormones in your body. You may also need to urinate a lot more often than usual -- your growing uterus is pressing on your bladder, and hormones are affecting the balance of fluid in your body. All will diminish as your pregnancy continues.
Exercise is good for you. Check out our fitness section to find out what forms of exercise are safe during pregnancy.
Fetal development - 8 weeks pregnant
This week, many changes take place in your baby, who now measures about 1.6cm. His embryonic tail is just about gone, and all his organs, muscles and nerves are beginning to function. In his brain, nerve cells are branching out to connect with one another, forming primitive neural pathways. Your baby's hands now bend at the wrist, and his feet are starting to lose their webbed appearance. His eyelids cover more of his eyes and tastebuds are forming on his tongue.
Your pregnancy is changing your figure as well. Your breasts may have grown large enough that you'll need bigger bras with better support than your old ones. You may notice your waistline expanding as well, forcing you to pack away your favourite jeans until next year.
Check out what to eat and drink throughout your pregnancy to make sure your baby gets all he needs to help him grow. Now's a good time to check out your maternity rights so you can plan ahead.
Fetal development - 9 weeks pregnant
By the end of this week, your baby measures about 2.3cm in length and weighs less than 2g. His eyelids, now completely covering his eyes, are fused and won't open until week 26. Tiny earlobes are now visible.
Your baby's essential body parts are accounted for, though they'll go through plenty of fine-tuning in the coming months. Already he looks more like a tiny human being. His wrists are more developed, his ankles have formed, and his fingers and toes are clear to see. His arms are growing longer and bend at the elbows. Now that your baby's basic physiology is in place, he's poised for rapid weight gain.
Though it's not yet possible to tell the sex of your baby by ultrasound, his genitals have begun to form. By now the placenta has developed enough to support most of the important job of producing hormones. The placenta is also making nutrients for your baby and getting rid of his waste products.
Around now, you should have your first antenatal appointment, called your booking appointment. This is when your midwife will take a blood sample for routine tests. Find out what you are being testing for. There is plenty more information about what to expect throughout your pregnancy in our complete guide to tests and care.
Fetal development - 10 weeks pregnant
Congratulations! Your embryo is now called a fetus, which means 'offspring'. She is about 3.1cm long from crown to rump and weighs less than 4g. She may be small, but she's very active, swallowing fluid and kicking her new limbs.
Her vital organs – liver, kidneys, intestines, brain and lungs – are fully formed and functional, while her head is almost half the length of her entire body. Her forehead temporarily bulges and sits high on her head, but later will change into a more baby-like feature. If you could take a look at your baby, you'd be able to see some exciting details, such as fingernails and peach-fuzzy hair. The outline of her spine is clear to see, too. Spinal nerves stretch out from the spinal cord.
And it's not just your baby who's growing. Your uterus (womb) is now the size of a grapefruit. You may even be able to feel it above the middle of your pubic bone.
Fetal development - 11 weeks pregnant
Measuring from the crown of his head to his rump, your baby is now about 4cm long. He has all his parts, from tooth buds to toenails and his fingers and toes have fully separated. Your baby is busy kicking and stretching. His movements are so fluid they look like water ballet. These movements will become more frequent as his body grows and becomes more refined.
Most of your baby's critical development will tail off in the next couple of weeks. His main task during the next six months will be to grow larger and stronger, until he can survive on his own outside your uterus (womb).
You're getting close to the end of the first trimester. A dark vertical line of pigmentation, called the linea nigra, may appear on your belly. Perhaps you are starting to find sleeping difficult? Find out why you're getting disturbed nights.
Fetal development - 12 weeks pregnant
Your baby is about 5.4cm long from her crown to her rump and weighs slightly less than 14g. The big news this week is that your baby's reflexes are becoming more honed. She'll squirm if you prod your belly, although you probably can't feel her moving around yet. Fetal nerve cells have been multiplying rapidly and the neurological connections in her brain (synapses) are forming. She can close her fingers, curl her toes and clench her eye muscles.
All the while, your baby's face is beginning to look more human. Her eyes, which started out on the sides of her head, have moved closer together. Her ears are almost in their final positions on the side of her head. Inside her body, her organs are performing more complex tasks. Her liver is making bile and her kidneys are secreting urine into her bladder.
Your midwife can now feel the top of your uterus (womb), also known as the fundus, low in your belly. As you approach your second trimester, you may need to wear looser clothing. Soon you won't be able to fit into your old clothes any more.
It's never too soon to start doing your pelvic floor exercises. A strong pelvic floor will protect you against urine leaks, and help with labour and birth when the time comes.
You may now find that once the sickness feeling has passed, your appetite returns. Find out how to eat well over the next few weeks. And if you're wondering how often you'll need to see your midwife or GP, take a look at our antenatal appointments scheduler.
Fetal development - 13 weeks pregnant
Your baby is now about 6.7cm long from crown to rump and weighs nearly 23g, about the same as a pea pod. Tiny fingerprints are now at the tips of her fingers. Your baby has sucking muscles in her cheeks, so when you poke your tummy gently she will feel it and start rooting. It's a rehearsal for the important instinct of searching for your nipple once she's born.
If you're having a girl, she now has approximately 2 million eggs in her ovaries. She will have only a million by the time she's born. She'll have fewer eggs as she gets older, and by age 17, the number will have dropped to 200,000.
As for you, the fog may be lifting, so to speak. The side-effects of early pregnancy, frequent trips to the loo, tiredness and nausea, diminish some time in the second trimester. Your second trimester starts when you've completed 13 weeks of pregnancy. You may not even feel like you're pregnant any more. But your increasingly rounded tummy will reassure you that you are!
Got lots of questions? Try chatting with other mums-to-be on our birth clubs and find out how others are feeling at your stage of pregnancy.
Fetal development - 14 weeks pregnant
Crown to rump, your baby is about 8cm long and weighs about 40g. Her body is now growing faster than her head. This week, her parchment-thin skin is starting to cover itself with ultra-fine, downy hair (lanugo) that usually disappears before birth. Though her eyebrows are beginning to grow and the hair on top of her head is sprouting, this hair may change in both texture and colour after birth.
Thanks to brain impulses, your baby's facial muscles are getting a workout as her tiny features form one expression after another. About now, your baby can grasp, squint, frown, and grimace. She may even be able to suck her thumb. Meanwhile, your baby's kidneys are producing urine, which she releases into the amniotic fluid around her. She'll keep this up until birth.
You should have had your first two ultrasound scans by the end of this week. A dating scan is offered between 11 weeks and 13 weeks plus six days. And a nuchal translucency scan, which checks whether your baby is likely have Down's syndrome, is offered between 11 weeks plus two days and 14 weeks plus one day. If you are over 35, have a look at our article on how age affects pregnancy.
Fetal development - 15 weeks pregnant
Although you've probably gained between 2.2kg and 4.5kg, your baby weighs in at only about 70g. He measures nearly 9.1cm long, crown to rump. You may not know it when it happens, but your tiny tenant frequently gets the hiccups, which babies learn to do before breathing. Babies don't make any sound because their windpipe (trachea) is filled with fluid rather than air.
Your baby's legs are growing longer than his arms now and all his joints and limbs can move. His genitals may be developed enough to show on an ultrasound scan. You'll have to wait a little longer, though, to find out if you're expecting a boy or a girl. Your next scheduled scan is at about 20 weeks.
This middle stage of pregnancy is a good time to start exercising. Visit our pregnancy fitness area for some work-out ideas and advice.
You may want to think ahead and book antenatal classes. Whether you are a first-time mum or want a refresher course, you're bound to benefit from a structured class. No matter what the philosophy behind it, a class can help prepare you for the rigours of labour and birth. You should plan to have completed the classes by 37 weeks. From 37 weeks, labour could start at any time and your baby will be considered full-term when he's born.
Fetal development - 16 weeks pregnant
Your baby is now some 10cm long from crown to rump, about the size of an avocado, and weighs about 100g. In the next three weeks she'll go through a tremendous growth spurt, doubling her weight and adding centimetres to her length.
In or out of the uterus (womb), babies are playful creatures. Yours may already have discovered her first toy, the umbilical cord, which she'll enjoy pulling and grabbing. Sometimes she may even clutch it so tight that less oxygen gets through. She'll let go of the cord before she starts to go without what she needs.
Your baby's head is more erect than it has been, and her eyes have moved closer to the front of her head. Her circulatory system and urinary tract are in full working order, and she's inhaling and exhaling amniotic fluid through her lungs. Your baby is immersed in amniotic fluid, which is protecting her as she grows in the amniotic sac.
Sometimes, when you move suddenly, you may a feel a slight pain in your sides. Ligaments on each side of your uterus and pelvic walls are stretching as your baby grows. It's normal to feel some pain, but if it continues for a few days or gets worse, talk to your midwife.
continues....
By the end of this week, your baby measures about 2.3cm in length and weighs less than 2g. His eyelids, now completely covering his eyes, are fused and won't open until week 26. Tiny earlobes are now visible.
Your baby's essential body parts are accounted for, though they'll go through plenty of fine-tuning in the coming months. Already he looks more like a tiny human being. His wrists are more developed, his ankles have formed, and his fingers and toes are clear to see. His arms are growing longer and bend at the elbows. Now that your baby's basic physiology is in place, he's poised for rapid weight gain.
Though it's not yet possible to tell the sex of your baby by ultrasound, his genitals have begun to form. By now the placenta has developed enough to support most of the important job of producing hormones. The placenta is also making nutrients for your baby and getting rid of his waste products.
Around now, you should have your first antenatal appointment, called your booking appointment. This is when your midwife will take a blood sample for routine tests. Find out what you are being testing for. There is plenty more information about what to expect throughout your pregnancy in our complete guide to tests and care.
Fetal development - 10 weeks pregnant
Congratulations! Your embryo is now called a fetus, which means 'offspring'. She is about 3.1cm long from crown to rump and weighs less than 4g. She may be small, but she's very active, swallowing fluid and kicking her new limbs.
Her vital organs – liver, kidneys, intestines, brain and lungs – are fully formed and functional, while her head is almost half the length of her entire body. Her forehead temporarily bulges and sits high on her head, but later will change into a more baby-like feature. If you could take a look at your baby, you'd be able to see some exciting details, such as fingernails and peach-fuzzy hair. The outline of her spine is clear to see, too. Spinal nerves stretch out from the spinal cord.
And it's not just your baby who's growing. Your uterus (womb) is now the size of a grapefruit. You may even be able to feel it above the middle of your pubic bone.
Fetal development - 11 weeks pregnant
Measuring from the crown of his head to his rump, your baby is now about 4cm long. He has all his parts, from tooth buds to toenails and his fingers and toes have fully separated. Your baby is busy kicking and stretching. His movements are so fluid they look like water ballet. These movements will become more frequent as his body grows and becomes more refined.
Most of your baby's critical development will tail off in the next couple of weeks. His main task during the next six months will be to grow larger and stronger, until he can survive on his own outside your uterus (womb).
You're getting close to the end of the first trimester. A dark vertical line of pigmentation, called the linea nigra, may appear on your belly. Perhaps you are starting to find sleeping difficult? Find out why you're getting disturbed nights.
Fetal development - 12 weeks pregnant
Your baby is about 5.4cm long from her crown to her rump and weighs slightly less than 14g. The big news this week is that your baby's reflexes are becoming more honed. She'll squirm if you prod your belly, although you probably can't feel her moving around yet. Fetal nerve cells have been multiplying rapidly and the neurological connections in her brain (synapses) are forming. She can close her fingers, curl her toes and clench her eye muscles.
All the while, your baby's face is beginning to look more human. Her eyes, which started out on the sides of her head, have moved closer together. Her ears are almost in their final positions on the side of her head. Inside her body, her organs are performing more complex tasks. Her liver is making bile and her kidneys are secreting urine into her bladder.
Your midwife can now feel the top of your uterus (womb), also known as the fundus, low in your belly. As you approach your second trimester, you may need to wear looser clothing. Soon you won't be able to fit into your old clothes any more.
It's never too soon to start doing your pelvic floor exercises. A strong pelvic floor will protect you against urine leaks, and help with labour and birth when the time comes.
You may now find that once the sickness feeling has passed, your appetite returns. Find out how to eat well over the next few weeks. And if you're wondering how often you'll need to see your midwife or GP, take a look at our antenatal appointments scheduler.
Fetal development - 13 weeks pregnant
Your baby is now about 6.7cm long from crown to rump and weighs nearly 23g, about the same as a pea pod. Tiny fingerprints are now at the tips of her fingers. Your baby has sucking muscles in her cheeks, so when you poke your tummy gently she will feel it and start rooting. It's a rehearsal for the important instinct of searching for your nipple once she's born.
If you're having a girl, she now has approximately 2 million eggs in her ovaries. She will have only a million by the time she's born. She'll have fewer eggs as she gets older, and by age 17, the number will have dropped to 200,000.
As for you, the fog may be lifting, so to speak. The side-effects of early pregnancy, frequent trips to the loo, tiredness and nausea, diminish some time in the second trimester. Your second trimester starts when you've completed 13 weeks of pregnancy. You may not even feel like you're pregnant any more. But your increasingly rounded tummy will reassure you that you are!
Got lots of questions? Try chatting with other mums-to-be on our birth clubs and find out how others are feeling at your stage of pregnancy.
Fetal development - 14 weeks pregnant
Crown to rump, your baby is about 8cm long and weighs about 40g. Her body is now growing faster than her head. This week, her parchment-thin skin is starting to cover itself with ultra-fine, downy hair (lanugo) that usually disappears before birth. Though her eyebrows are beginning to grow and the hair on top of her head is sprouting, this hair may change in both texture and colour after birth.
Thanks to brain impulses, your baby's facial muscles are getting a workout as her tiny features form one expression after another. About now, your baby can grasp, squint, frown, and grimace. She may even be able to suck her thumb. Meanwhile, your baby's kidneys are producing urine, which she releases into the amniotic fluid around her. She'll keep this up until birth.
You should have had your first two ultrasound scans by the end of this week. A dating scan is offered between 11 weeks and 13 weeks plus six days. And a nuchal translucency scan, which checks whether your baby is likely have Down's syndrome, is offered between 11 weeks plus two days and 14 weeks plus one day. If you are over 35, have a look at our article on how age affects pregnancy.
Fetal development - 15 weeks pregnant
Although you've probably gained between 2.2kg and 4.5kg, your baby weighs in at only about 70g. He measures nearly 9.1cm long, crown to rump. You may not know it when it happens, but your tiny tenant frequently gets the hiccups, which babies learn to do before breathing. Babies don't make any sound because their windpipe (trachea) is filled with fluid rather than air.
Your baby's legs are growing longer than his arms now and all his joints and limbs can move. His genitals may be developed enough to show on an ultrasound scan. You'll have to wait a little longer, though, to find out if you're expecting a boy or a girl. Your next scheduled scan is at about 20 weeks.
This middle stage of pregnancy is a good time to start exercising. Visit our pregnancy fitness area for some work-out ideas and advice.
You may want to think ahead and book antenatal classes. Whether you are a first-time mum or want a refresher course, you're bound to benefit from a structured class. No matter what the philosophy behind it, a class can help prepare you for the rigours of labour and birth. You should plan to have completed the classes by 37 weeks. From 37 weeks, labour could start at any time and your baby will be considered full-term when he's born.
Fetal development - 16 weeks pregnant
Your baby is now some 10cm long from crown to rump, about the size of an avocado, and weighs about 100g. In the next three weeks she'll go through a tremendous growth spurt, doubling her weight and adding centimetres to her length.
In or out of the uterus (womb), babies are playful creatures. Yours may already have discovered her first toy, the umbilical cord, which she'll enjoy pulling and grabbing. Sometimes she may even clutch it so tight that less oxygen gets through. She'll let go of the cord before she starts to go without what she needs.
Your baby's head is more erect than it has been, and her eyes have moved closer to the front of her head. Her circulatory system and urinary tract are in full working order, and she's inhaling and exhaling amniotic fluid through her lungs. Your baby is immersed in amniotic fluid, which is protecting her as she grows in the amniotic sac.
Sometimes, when you move suddenly, you may a feel a slight pain in your sides. Ligaments on each side of your uterus and pelvic walls are stretching as your baby grows. It's normal to feel some pain, but if it continues for a few days or gets worse, talk to your midwife.
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why every pregnant women said during prgnency feel so problems?
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