Stages of Growth - Fetal Development

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Stages of Growth - Fetal Development

Did you just find out that a little soul is growing in you? You're curious to know every single details about it, aren't you?‌ Then you are absolutely coming to the right place. Here we talk about how your baby grows in your womb week by week during pregnancy. Definitely you are also keen to know how your baby makes the way from a fertilized egg to a full-fledged human child. Then let's get started.


How Does Fertilization Take Place?


In a word, fertilization results from sperm and ovum fusion. In the female's body at the time of ovulation egg or ovum release from the ovaries and waits for sperm. After the intercourse, when male partner ejaculates semen into the female's vagina, millions of sperm swims towards the egg. One lucky swimmer reaches the egg first and fuses, this is called fertilization. Now the fertilized egg comes down to the uterus and implants. Next development stages take place.


When to Have the Pregnancy Test?


When to have the pregnancy test

For accurate results, wait up to 1 week after you miss your periods and then test. But if you don't want to wait as long, after having intercourse during ovulation, wait for 4-5 days and then have a pregnancy test.


When to See the Doctor?


When you have a positive result in a pregnancy test, you should consult the doctor. Go to the clinic and have a blood test to confirm your pregnancy. Most women go to the doctor at least after 8 weeks but consulting with your doctor from the first is best for you.


What is EDD?


What is EDD

Estimated date of delivery or estimated due date (EDD) is a term describing estimated delivery date of that pregnant woman. A normal pregnancy lasts from 38 weeks to 42 weeks. On average childbirth occurs in the middle of this time, at the gestational age of 280 days or 40 weeks. To estimate the due date, experts consider the last menstrual period (LMP) as the first week of pregnancy and after 40 weeks from LMP they suggest the due date. There are also many ways to estimate the due date.




The whole pregnancy divides into three parts to fetal development, each part is called a trimester. There are three trimesters in a pregnancy.

From week 0 to week 13 is the first trimester where the early fetal development takes place. This is the most vital phase of pregnancy as most of the miscarriages occur at this time.

From week 14 to week 26 is the 2nd trimester where the development of the baby speeds up the most and in this phase most of the organs of the baby starts to form.

From week 27 to week 42 is the third trimester where the baby fully develops and prepares for the birth.


What is referred to by the terms zygote, embryo and fetus?


What is referred to by the terms zygote, embryo and fetus

Before describing the developmental stages of your baby let us give you an idea about the following terms.

  • Zygote: After fertilization, when the sperm and egg fuses the fertilized egg is called a zygote which leads the division.

  • Embryo: After the zygote implants into the uterus it's called an embryo.

  • Fetus: After 9 weeks of conception or after 11 weeks of pregnancy the embryo is called a fetus.


What is Placenta?


Placenta is a temporary fetal membrane which develops during pregnancy. It is attached to the wall of the uterus and provides nutrients and oxygen to the growing baby. It removes wastages and develops umbilical cord.


What is Umbilical Cord?


Umbilical cord is a tube-like structure which connects the baby to the placenta. It includes one vein and two arteries. The vein carries the food and oxygen from the placenta to your baby and the arteries carry the waste materials from the baby to the placenta. Some parents choose to donate their child's umbilical cord as it reaches stem cells that can help many patients suffering from life threatening disease.


What is an Amniotic Sac?


Amniotic sac is a thin-walled sac-like structure that surrounds your baby during pregnancy. The sac is filled with fluid called amniotic fluid. The amniotic sac protects the baby (fetus) from injury and also helps to regulate the body temperature of it.


Development of a baby inside your womb week by week:


First trimester (week 0 to week 13):


Week 1:

This is the week of your last month period, you are not pregnant yet. To calculate the due date doctors consider this week as your 1st week of pregnancy. This week your body prepares for ovulation.


Week 2:

This is the week of your ovulation. This time after the intercourse the male partner ejaculates semen into the female's vagina. Millions of Sperms swim to reach the egg. One lucky swimmer reach the egg first and fertilized the egg.

Now the fertilized egg has 23 chromosomes from you, 23 chromosomes from your partner and carries all the genetic information. Now your baby, known as the zygote, travels down towards the uterus. The zygote starts to divide for further development.

If you are having multiple babies then the egg splits in two, and forms identical or monozygotic twins. If two of your eggs are fertilized by two sperm then you have non-identical or fraternal or dizygotic twins.


Week 3:

This week the zygote divides and forms a blastocyst. The cells of blastocyst clump together and become an embryo.

The embryo attaches to the outer lining of your uterus for further development, this process is called implantation. You may bleed a little during implantation.


Week 4:

In this week your baby develops the placenta and fluid filled amnion. These organs protect your baby and give nourishment.

If you are having monozygotic twins then they develop a single placenta and two amnions for each. If you are having fraternal twins then they develop separate amnions and separate placentas for each.


Week 5:

Now the embryo takes its distinct shape and develops three germinal layers called ectoderm, endoderm and mesoderm. If you forgot your life science lessons then remind you that from these three layers all the organs and systems are formed. From the ectoderm nervous system, epidermis and other tissues are developed. From the endoderm gut and all the internal organs are formed. From the mesoderm muscle cells and all the connective tissues are developed.

In this week your baby develops the heart, neural tube and blood vessels.


Week 6:

In this week as your little's heart completely forms, it starts beating. At this time your baby's heart beats 150 times in a minute. In fact the heartbeat rate is double compared to the adults.

In this week your baby is also developing digestive and respiratory systems. The small bud-like structure that becomes the hands and legs starts appearing. Brain hemisphere also develops.

That means if you have an ultrasound after one or two weeks your baby's heartbeat and the brain waves will also be noted.


Week 7:

Your baby's facial features are also developing; mouth, ears, eyes, nose are taken shapes. Baby's arm buds now develop a paddle-like hand on the end of it.

The nerve cells are growing at the rate of 100,000 cells per minute.


Week 8:

Your baby is now quadrupled in size.

This week toe buds and fingers buds start growing and the ears, upper lips, nose, eyelids develop.


Week 9:

Now your little one measures about an inch long.

As muscles are developed your baby may make the first movement but you won't feel them.

Organs like pancreas, gallbladder, reproductive organs are developing.

At this time the head of your baby is growing bigger so that the head covers half of the length of the body and is tucked towards the chest.


Week 10:

Developing very fast at the rate of 250,000 neurons per minute.


Week 11:

Until now your baby was classified as an embryo but from the 11th week it is a fetus.

From this week onward, most of the birth defects start rising.

The fetus starts to inhale and exhale the amniotic fluid to develop the lungs.


Week 12:

This is the end of the first trimester and congratulations to you, your baby pases the most vital phase of development.

Your baby's tiny fingernails and toenails start to form.


Week 13:

This is the start of the second trimester. This week your baby's ears start moving from the neck into the right place and eyes move from the side of the head to the front of the face.

This week your baby's little arms develop completely and lengthen proportionate with body.

Your baby starts moving and kicking as well. Your baby may be able to put a thumb in mouth but you might not feel it yet.


Week 14:

This week your baby's genitals are fully developed. If your baby is a boy then his prostate is developed and if a girl, her ovaries are moving down into the pelvis.


Week 15:

Your baby's eyebrows and hair start to grow. Day by day your baby is looking more like a cute doll.

As the facial features are developed now the little can frown, squint, grimace and wince also by practicing the facial expressions.


Week 16:

Some fine colourless hairs start developing in your baby's body, mostly on the face, back, shoulders, ears. This hair is called lanugo. It protects the skin and helps to retain body heat but when the right amount of fat is gained to do the job, it falls off before delivery.

The umbilical cord is fully developed with one vein and two arteries.

If your baby is a girl then thousands of eggs start forming in her ovaries for your future grandchildren.


Week 17:

Your baby is about 5 inches long and weighs about 5 ounces.

As your baby is able to swallow the amniotic fluid for nutritions, may get hiccups also.

In this week fat deposition starts under the baby's skin and sweat glands also develop.


Week 18:

From week 16 to 18 the bones of the baby begin to harden or ossify. The delicate skeleton develops from rubbery cartilages to bones.

Some mothers begin to feel their baby's movements at this time.


Week 19:

A waxy, cheese-like white substance called vernix caseosa is covering your baby's skin to protect it from being chapped or scratched. When the baby is born most of the vernix caseosa is gone, if not then it will be washed off after your baby's first ever bath.

Your baby's heartbeat is growing stronger.


Week 20:

Congrats you're halfway through your pregnancy.

Your baby's well developed arms and legs continue to explore curling, flexing and kicking.

If you haven't felt it already, you might feel your baby's first kick at this time.


Week 21:

This week your baby's ears are moved into their final position and probably starting to hear also.

So, start talking to your baby, sing songs as probably the little one starts to hear you.


Week 22:

This week the taste buds and sense of touch is developing. It means your baby now starts to taste the amniotic fluid and also touches the toes.

The see-skin of your baby is now becoming opaque.


Week 23:

As billions of brain cells develop, these all control the baby's movements and sensory.

Now the baby's daily routine is to move the muscles of fingers, toes, arms and legs.


Week 24:

This week your baby's lungs are developed so the baby will start practicing breathing movements but still relies on the placenta for oxygen.


Week 25:

As your baby's hearing continues to develop, he or she is able to hear your voice, music and the voices of others close by.

As the sensory organs develop your baby might be able to respond to your voice or the rubbing of your belly. A sudden loud sound or a tao on the belly alerts your baby.

In this week the balance controlling inner ears fully formed so finally your baby can feel the position and moving around takes new meanings.


Week 26:

This is the last week of the second trimester.

The first bowel movement of your baby's also forming in the large intestine. After birth a thick black stool excretes, it's called meconium.


Week 27:

This is the first week of the third trimester.

Your baby settled in a distinct sleep-wake pattern, you might feel that sometimes your baby is very active kicking or punching and sometimes sleep.

Your baby's lungs, liver and other systems still need to fully develop but if for some issues your baby is born then there is a good chance of its survival. There are many cases where babies are born prematurely at this time and survive well.


Week 28:

The little eyes that are sealed for very long, are just beginning to open and your baby will get the first glimpses of the womb. The irises are filled with pigments.


Week 29:

Your baby's brain, associated with intelligence and personality, becomes far more complex that now the little one can react to different food, sounds and lights.

Your baby is also gaining a lot of weight during this time.

You might feel a jerky motion or twitches in the uterus during this time which is caused by your baby's hiccups as it swallows the amniotic fluid. A baby gets hiccups several times a day. But if the hiccups are more than usual then please contact your doctor.


Week 30:

From this week your baby will gain about half pound each week. As the baby, by adding fats, grows bigger, you might find that the fetal movements become less.

But if you do not feel any jabs or occasional kicks for a long time then contact your doctor.


Week 31:

At this time your baby pees several cups of urine per day in the amniotic fluid.

If your little one is a boy then his testicles are moving through the groin into the scrotum and if a girl then her clitoris is already formed.


Week 32:

This week your baby weighs about 4 pounds. By this time all the major organs are fully functioning except the lungs. The lungs need a little more time.

Lanugo, the fine hairlike structure now starts to fall off as the body is gaining enough fat to retain the body heat.


Week 33:

Your baby becomes less active as your baby doesn't have much room these days, space in your uterus is running out.

Your baby sleeps most of the time and has REM (rapid eye movement) during which dreams are caused.


Week 34:

As the delivery date comes up the bones that make the baby's skull start moving inside the pelvis, this is called molding. It helps to pass through the birth canal.

Vernix caseosa begins to thicken and ensures that during birth your baby is protected. Your baby will arrive covered in that but after the first ever bath of your baby it will all be washed off.


Week 35:

At this point your baby is now cramped inside the uterus and should be positioned with its head facing the cervix and vagina. Most of the babies (about 97%) adopt this position. If your baby remains in breech position when the butt or the feet come out first then the chances of your c-section increases. But your doctor can solve this problem by correcting the position of your baby before birth. Your health provider can apply pressure on your belly that makes your baby turn around to the right position.


Week 36:

This week your baby is probably in the position of delivery and preparing for birth.

The fully developed sucking muscles are absolutely ready for the first real feeding. Soon after birth your baby becomes hungry.


Week 37:

In most of the cases the labour pain starts after this week. So be prepared that your baby is coming very soon.

Your baby's eyesight is so developed now that if a bright light shines on your belly, the baby may see it and also turn towards it.

The umbilical cord passes antibodies to your baby in preparation of birth so that your baby will better prepare for germs and diseases outside the worm.


Week 38:

As your baby drops into the pelvis, your bladder is squished, making you pee more.

Your baby is enjoying the last few weeks in your womb. Your baby's senses are fully developed so that now the little one can hear and recognise your voice. Soon after birth when he or she may hear you, able to turn the head towards you. Your baby knows you by your voice.

Your baby's fingers are more coordinated now that after birth they are able to grab your fingers also.


Week 39:

This week your baby now reached the birth weight (typically between 6 and 9 pounds) and length (18 to 22 inches from head to toe). Your baby is absolutely ready to arrive.


Week 40:

This may be the last week of your pregnancy. Very soon you will meet your baby for the first time. If the baby doesn't come by the end of this week, then there is nothing to worry about. In many cases the baby arrives between 40-42 weeks it's normal. Maybe your baby wants to be there for a few more days.


We describe here how the baby develops from a ball of four cells to a human child.


For the full development of the unborn child, they need the nutrients they get from the mother's diet. So we want to tell all expectant mothers to leave stress and be happy and eat well. Your baby is also very keen to meet you. Your baby will be healthier if you have preconception care before conceiving. To know more about Preconception care visit our previous blog about Preconception care.

Piyali Das

Piyali Das