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The pregnancy is divided into 3 trimesters (first trimester is from week 0 to week 13th, second trimester is from week 14th to week 26th and third trimester is from week 27th to week 40th). We will be covering the details of foetus growth week by week.

Human embryonic and foetal development across weeks of pregnancy / week by week growth of baby during pregnancy

First trimester


WEEK 1 & 2 | Pregnancy Made Possible

Pregnancy is usually calculated from the first day of menstruation of the last period before pregnancy. During this week, the ovary is still preparing to release the egg which will later fuse with the sperm to form the zygote which results in the pregnancy. The uterine lining, during this week, is shedding because of menstruation. However, it will continue to thicken after the menstrual discharge of blood is over.

Day 12th to 14th of the menstruation cycle (in a cycle of 28 days, varies otherwise) are the most fertile days. This is when the woman ovulates. The egg is released into the fallopian tube by the ovary. If any sperm is present in the fallopian tube, it will fuse with the egg and result in the pregnancy. So the zygote (baby) is formed in around 3rd week of menstruation cycle. 

WEEK 3 | Fertilisation, Formation of Zygote

It is by the 3rd week that the sperm and the ovum (egg) have fused to form the zygote. The zygote continues to divide to form multiple celled structures. This multi-celled structure is also known as an embryo and it continues to travel down the fallopian tube to the uterus to implant in the endometrial lining of the uterus.

The zygote (like most humans) has 46 chromosomes - 23 from mother and 23 from father. These chromosomes help in determining the baby's sex and traits such as eye and hair colour, and, to some extent, personality and intelligence. 

WEEK 4 | Implantation

By this week, the zygote has been implanted into the uterus. 

This implanted zygote continues to grow in size and forms a structure with two parts known as the amniotic sac and the yolk sac. The amniotic sac is a fluid cushion which saves the baby from any injury and shocks.  The yolk sac and tissues in the uterine lining will provide nutrition to the baby at this stage. The baby is barely the size of a rice grain at this stage.

Growth of fertilized egg to zygote, blastocyst and the embryo

WEEK 5 | Embryo Structure Developing

The embryo continues to grow and form structures which lead to the development of the most vital organ systems of the body- the nervous system and the heart. The tube-like structures which forms the brain, spinal cord and heart makes the embryo look like a tiny collection of tubes with four small outgrowths which will form the arms and legs of the developing embryo. 

This is when the mother starts to feel the first symptoms of pregnancy. 

WEEK 6 | Size of a Tadpole

The heart of the embryo is now functional and the embryo is now the size of a tadpole (1.5 inches). 

In an ultrasound, the embryo will look like a tadpole. The beats of the heart are too faint to be heard during this week. The development of the eyes begins, along with the digestive and respiratory system.

WEEK 7 | Functionality of Umbilical Cord 

By this time, the umbilical cord is developed and the placenta is in the process of being fully functional. The sense organs continue to develop and the other organ systems like the urinary system, the liver, kidney, intestines also begin to develop. The nutrition required for the metabolic processes of the baby is provided by the placenta and the umbilical cord. Umbilical cord will also remove the excreta/body waste of the foetus from the uterus. 

WEEK 8 | Foetus

The mother begins to experience tiredness and morning sickness in some cases due to the development of the baby. The spine of the baby is almost visible and helps in straightening baby's body. The sense organs like eyes, ears, nose and lips start to become visible. The face of the baby is distinguishable from rest of the body and from this stage onwards, the baby is called a foetus and not an embryo.

WEEK 9 | Weight of a Coin

The foetus now develops legs and its head is visibly larger than the rest of the body. The reproductive organs are developing, but the gender cannot be distinguished during this stage. The weight of the foetus is about the weight of a coin.

WEEK 10 | Signs of Movement

The foetus now has a noticeable pair of legs, along with fully developed eyes, ears and mouth. The hands and feet are growing and they are not webbed anymore. The development of the neural networks in the brain continues during this stage and the foetus may show certain signs of movement.

WEEK 11 | Size of a Date

The size of the foetus continues to grow and is now reaches the size of a date. The fingers are completely developed and now have fingernails developing on them. Moreover, the development of the eye continues as the iris of the eye begins to develop.

WEEK 12 | Unique Body Parts

The fingernails at this stage are fully developed in the foetus. The foetus is about to have a functional urinary system and shall begin producing tiny quantities of urine later. The foetus looks very distinctive due to its unique body parts.

WEEK 13 | Pregnancy Bump

The foetus now looks more proportional as the rest of the body has grown significantly and the head is no longer too large. The head just covers one-third of the length of the foetus. The placenta is fully developed by now and provides nutrition and oxygen to the baby through the umbilical cord and drains out the waste from the foetus’s body. The baby bump starts to appear in the mother now.


Second trimester

WEEK 14 | Size of a Peach

The foetus is now the size of a peach. Tiny hair has begun to develop on its body which in turn keeps the foetus warm.  These are known as lanugo.  In case of a girl baby, the ovaries start producing eggs. Also, now the fingerprints of the foetus are developing while the umbilical cord and placenta are fully functional.

WEEK 15 | Muscle Development

The blood vessels and muscles of the foetus are beginning to develop. The skin of the foetus is extremely thin, thus some of the internal structures might be visible during an ultrasound. The foetus begins to move around due to muscle development and begins moving its hands by making fists.

WEEK 16 | Beginning of Blood and Nutrient Supply

The chromosomal test for any genetic disorder like down’s syndrome is recommended to be checked during this week, by the obstetrician (doctor). The foetus is now about 5 inches long and requires rigorous blood and nutrient supply through the placenta. This results in increased blood volume in mother to provide for the requirement of the foetus. The increased blood volume causes pinkish skin leading to pregnancy glow in mother. Sometimes, it also leads to enlarged leg veins and occasional nose bleed in mother.  

WEEK 17 | The Functioning Foetus

All the systems in the foetus’ body are functioning now, except for the lungs which will start working only after birth. The foetus is circulating blood and filtering it in the kidney to produce tiny quantities of urine. The eyelashes, hair and eyebrows of the baby begin to grow.

WEEK 18 | Size of a Tiny Notepad

The size of the foetus is about 5.5 inches long: the size of a tiny notepad. This increase in size and development of stronger muscles causes the foetus to move in the amniotic fluid. Movements of the baby will be felt by the mother during this stage. The sex organs of the baby have fully developed.

WEEK 19 | Vernix Caseosa, The Waxy Coating

The baby’s body is covered by a waxy coating called vernix caseosa which protects the baby from any impurities present in the amniotic fluid. This layer will be shed before birth, unless it's a pre-mature delivery. A layer of subcutaneous fat begins to develop inside the baby's skin to provide warmth. At this point, the lanugo hair slowly begins to reduce and the baby’s hair on his/her scalp begins to grow.

WEEK 20 | Size of a Banana

The foetus is 6.5 inches long: about the size of a banana. The sense organs of the baby are fully developed and it can hear certain frequencies. The foetus continues to grow and the mother puts on a significant amount of weight. Besides, this is the perfect time for the first ultrasound, where the parents can view their baby for the very first time and the doctor can throughly check for any concerns in the pregnancy.  

WEEK 21 | Increase in RBCs in the Mother

The tooth buds start to develop in the foetus. The bone marrow starts producing more Red Blood Cells, which will be required to deliver oxygen to the baby’s cells. The intestine of the baby is currently producing a black, sticky excreta known as meconium which shall be excreted after birth and is greenish in colour. Some babies may pass meconium while still in the womb during late pregnancy. At this stage, some rapid eye movements also begin to occur.

WEEK 22 | Beginning of development of Nerves and Hormones

The size of the foetus is about 8 inches. The nerves of the nervous system and the hormones of the endocrine system are beginning to develop in the foetus. The sex organs continue to develop. In case of boys, the descent of the testes from abdomen begins. These testes shall settle in their scrotal sacs before childbirth.

WEEK 23 | Preparing Lungs to Function post Birth

The foetus is about 1 pound in weight now. The neural networks in the brain continue to develop and the lungs are preparing to breathe after birth. The alveoli in the lungs have begun to produce a chemical known as the surfactant which will help to maintain the surface tension in the lungs once the baby starts breathing after birth.

WEEK 24 | Life-support sustainable Foetus 

The foetus weighs more than a pound and is now sustainable on its own. 

That means in case of a premature delivery at this stage, the baby can survive with the help of life support outside the womb as the development of the supportive organ systems is almost complete. De-wrinkling of the baby’s skin begins at around this time as the subcutaneous fat layer begins to deposit beneath the skin. The fingernails are also fully developed by now. 

The test for gestational diabetes (diabetes during pregnancy) will be performed at around this time by evaluating the blood sugar level of the mother.

WEEK 25 | Audible Heart beats

The foetus continues to grow and weighs around 1.5 pounds. The heart is almost fully functional by now and heartbeat of the baby can be heard by pressing closely against the mother's stomach.

WEEK 26 | The deep Blue Eyes

The baby shows some movement and sensitivity to light as the eyelids have formed and opened completely. Here, the eyes will be of a deep blue colour, which is subject to change in the next few weeks.

Third Trimester

The position of the foetus in the uterus during the last trimester/baby's head moving towards cervix for delivery

WEEK 27 | The Kicks Felt 

The sleep schedule of the baby kicks in and it is not the same as the mother. Any kicks felt by the mother in the middle of the night is because the baby is moving in the amniotic fluid. The baby almost looks fully developed now but the neural networks in the brain need further time for development. The mother needs to make sure to take her recommended supplements of calcium and iron.

WEEK 28 | Starts shifting position for Birth 

The mother needs to visit gynaecologist/obstetrician more often from now i.e., at least one consultation every 2 weeks. This is because, the position of the baby needs to be monitored and since the baby is of considerable size now (10 inches long), any other complications like strangling by the umbilical cord need to be assessed frequently. 

The baby can blink its eyes, have hiccup and circulate blood. 

Also, the baby is slowly shifting into the position required for childbirth. If any such significant position shifts are not observed, alternate methods of childbirth shall be discussed with gynaecologist.

WEEK 29 | Significant Muscle Growth

The brain of the baby is developing and maturing as the grooves on the surface of the brain (known as gyri and sulci) continue to develop. Certain strong knee jerks and elbow jabs of the baby can be felt by the mother as the movements become stronger due to significant muscle growth.

WEEK 30 | Mother’s body starts preparing for Labour

The baby has grown up to 11 inches long and this increase in size can cause considerable changes in the mother, i.e., the centre of balance might tip and oedema or swelling in the feet becomes more significant. The body of the mother starts preparing for labour. Both mother and the baby starts putting up considerable weight from now on.

WEEK 31 | Braxton Hicks Contractions in Mother

Since the mother's body is preparing for labour, minor contractions known as Braxton Hicks Contractions begin to kick in which last for 30 seconds to 2 minutes at a stretch. However, if the contractions keep getting closer, longer and stronger; contacting the obstetrician is recommended to confirm if they are labour contractions. If premature birth occurs now, a ventilator will be required to support the baby to breathe as the lungs are not fully developed yet.

WEEK 32 | Movements felt way stronger

The baby weighs about 4 pounds by now and the lanugo hair on its body begin to shed. Only the hair in the eyebrows, scalp and eyelashes remain. The baby continues to move around in the amniotic fluid and these movements are felt strongly by the mother. Braxton Hicks contractions will continue until labour.

WEEK 33 | Extension of the Uterus

The bones in the baby begin to strengthen during this stage, except for the skull bones. There is a gap between the bones which make up the soft skull, also known as Fontanels. This exist because the size of the brain continues growing in size even after birth. 

The mother begins to get uncomfortable as haemorrhoids begin to develop and a sensation of heartburn is felt due to the pressure exerted by the extension of the uterus.

WEEK 34 | The head facing Mother’s Cervix 

The baby weighs about 5 pounds and has almost shifted to the delivery position, with the head facing downwards towards the cervix of the mother. The respiratory, digestive, urinary and nervous system of the baby is now developed.

WEEK 35 | The Lightning

The baby begins to shift further downwards in the uterus to prepare for delivery. This is known as lightning. This results in increased pressure on the bladder of the mother which then leads to frequent urination. However, due to this downward shift, gastric reflux should reduce and breathing should become easier for the mother.

WEEK 36 | Shedding of Waxy Covering 

The baby weighs about 6 pounds and is already taller than a foot. The waxy coating on its body, called the Vernix Caseosa, begins to dissolve and the baby may swallow a part of the shed coating. This forms the blackish, sticky liquid present in the child’s faecal excreta after birth, known as the meconium. The frequency of visiting the OB/ GYN should be further increased.

WEEK 37 | Full Term Pregnancy 

At a weight of around 6.5 pounds, the pregnancy is now considered to be full term. The head of the baby is moving further downwards in the pelvis of the mother. This is known as “engaging”. The immune system of the baby is preparing for the outside world by producing some elementary antibodies. Labour contractions are expected at any moment during or after this week.

WEEK 38 | Baby’s Reflexes Activated

The reflexes of the baby are now active and almost all the organ systems have developed by now. The process of engaging into the pelvis continues, while preparing for delivery.

WEEK 39 | Intense Contractions

The baby weighs about 7 pounds and is already too large for the uterus. The Braxton Hicks contractions will become more intense and longer and if they persist with frequency, the obstetrician should be called in order to decide whether or not to consider immediate preparation for the delivery.

WEEK 40 | Commencement of Labour

The baby is about 7 pounds in weight, 20 inches tall and this is the week where labour contractions should occur, leading to the delivery. If there is any delay in the process of delivery, labour can be induced by several methods and the options shall be discussed with the mother by the obstetrician. As soon as the pre-labour contractions begin, it is vital to rush to the hospital to prepare for the deliver of foetus.

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