Fetal Development Week by Week

Fetal Development Week by Week: Read how your baby is growing week by week during your pregnancy with fetus development.

 Dr Sonali Shivlani
PregnancyWritten by: Dr Sonali ShivlaniPublished at: Apr 22, 2013
Fetal Development Week by Week

Fertilized egg becomes a two-celled organism. This is the most rapid and fragile growth period of pregnancy. The fertilized egg, called a zygote, divides approximately 18-36 hours after fertilization to become a two-celled organism. By 48-72 hours, the zygote becomes a morula, consisting of 16 cells. While this growth is occurring, the cell cluster is moving through the fallopian tube to the uterus. In about four days, the morula, now 32 cells, reaches the ute

Bag of water or amniotic fluid. Fluid begins to fill the hollow sphere that has been created. This sphere, a blastocyst, consists of several layers of cells and floats for two to four days before attaching itself to the uterine wall. The outer layer of cells, trophoblasts, becomes the placenta. These cells attach to the uterine wall around day 7. The inner layer of cells becomes the embryo. The remaining space inside the blastocyst is filled with a fluid that becomes the “bag of water” or amniotic fluid.

Baby’s liver and digestive system starts to develop. Next, placenta and the maternal blood supply to the placenta begin to develop. The embryo becomes a two-layer structure, the endoderm and the ectoderm. The chorion, an outer membrane, is the foundation of a third layer, the mesoderm. The amnion cavity is established and a yolk sac develops. The yolk sac will develop into the baby’s liver and digestive system.

 By day 15-16 the three layers are complete. The three “germ” (root of the word germinate – to sprout, develop or grow from) layers, ectoderm, mesoderm, and endoderm, are the layers from which all the body systems are specifically derived.

Part of the ectoderm grows inward to form the foundation for the nervous system during weeks three and four. The rest of the ectoderm later forms the outer integumentary system (hair, skin, nails) and some of the endocrine system (glands that secrete hormones – such as thyroid).


The third week progress

During the third week, the mesoderm lays the foundation for the muscular system, skeletal system, cardiovascular system, lymphatic system and parts of the integumentary (skin, hair, nails), endocrine, respiratory, urinary and reproductive systems.

At the same time, the endoderm or inner layer is laying the foundation for the digestive system (food tube, stomach and bowel) and parts of the reproductive, urinary, respiratory, and endocrine systems.

The single chamber heart begins to beat by day 25. By day 28, the neural tube (spinal column) of the nervous system should be completely closed. The thyroid begins to develop and early muscle cells are formed. The gut begins to differentiate into the separate functioning parts of the digestive system (food tube, stomach and bowel). The beginning of the limb buds, lung buds, tongue bud, parathyroid, lens, thymus, the urinary and reproductive systems also starts to develop.

As you can see, so much occurs in the first month, as the foundation is established for further development in the months to come. The embryo is only about 1/2 of an inch long and weighs about an ounce and is truly amazing!


Month 2

During month two, the entire digestive system is formed. The stomach, liver, pancreas, gall bladder, spleen and intestines all blossom from the inner layer, the endoderm. The fetus is now about the size of a grape. The head and facial features have human characteristics, with closed eyelids, ears, and a mouth. The fetus has all limbs including arms, legs, fingers and toes, but they are supported by cartilage because the bone formation and hardening process is not complete. The external genitalia also forms and the lungs develop their separate lobes.

Month 3

The fetus has developed each body system, and will further mature these systems in the months to come. The kidneys are now functioning. Hair is growing. The fingers and toes are more prominent. Tooth buds are forming. The fetus is about 2-1/2 inches long and weighs a little more than an ounce

Month 4

The fetus’ heartbeat is now stronger and it has active periods during the day, alternating with rest and sleep. This becomes a predictable cycle of waking and sleeping. The fetus swallows the amniotic fluid and is able to eliminate this by urinating. The body is covered with a fine downy hair called lanugo, which protects the skin. Growth has been amazing and the fetus is now almost 3 times the size and 5 times the weight of last month -- measuring about 6 inches long and weighing about 5 ounces.

Month 5

At month five, your pregnancy has reached the halfway mark, and the signs of life are evident with a bulging tummy obvious from the outside. On the inside, the heart is beating and mom can feel the first fetal movements. These movements are first felt around week 20 of pregnancy. Early movements feel like a flutter of butterflies or big bubbles moving around. Most women find this very exciting – proof that you’ve got something special going on!


The fetal development

As far as fetal development, the lungs are now able to exhale amniotic fluid. A system of blood vessels and a pumping heart are working with great precision. The urinary system is functioning and making urine. The external genitalia are now distinctly male or female, and if the baby is in the right position, an ultrasound can usually identify the sex if you wish. The growth continues to be phenomenal with a weight of approximately 1/2-1 pound, and a length of 8-12 inches long.


Month 6

During month six, the fetus’ skin is becoming less transparent as the fetus steadily gains fat and the integumentary system (skin, hair, and nails) continues to mature. The internal organs are continuing to develop and become stronger. The fetus can now hear and becomes startled at loud bangs. Many women begin to listen to classical music or favourite tunes. Singing, or just the sound of mother’s voice, is now music to the delicate ears of this growing baby.

The top of your uterus is now well past your navel, and you are most decidedly looking pregnant!

The kicks, turns and twists are becoming stronger, and you might find it fun to just look at your bare belly and watch the activity.

Flaunt it and enjoy – the baby is now 11-14 inches long and weighs about 2 pounds. You look beautiful!

Month 7

Bones begin to harden in the process called ossification during this month. The eyelids are no longer fused together, and they can open and close. The hair, including eyelashes, is becoming thicker and darker. Growth is rapid as the fetus gains more fat – the weight is now about 3 pounds and the length is 15-16 inches.

Month 8


It’s getting close to the time you’ve been waiting for – the birth of the baby that has been growing these many months.


To prepare for this, the fetus turns to a head down position and snuggles into the pelvis. The space is getting a little cramped and rapid fluttery movements are fewer, it’s now big thumps and kicks. It is important to continue to feel daily periods of active movement – check with your doctor if you notice any change in the quality or quantity of fetal movements.


The baby is certainly ready for life outside the mother now – and, if born, most babies would be able to breathe air without any problem. The lungs have become stronger. The fetus is almost fully-grown, weighing about 5 pounds and measuring 18 inches in length.

Month 9


The weight gain for the baby at this point is about a 1/2 a pound a week. The uterus is almost at full capacity, and the baby has dropped down into the pelvic inlet or birth canal. The fetus will shed some of the protective lanugo (fine body hair) and cheese-like skin coating. To assist with getting labor started, the fetus produces high amounts of the hormones - oxytocin, cortisol and prostaglandin - around the time of delivery. These hormones stimulate contractions of the uterus. The average weight is 6-9 pounds and length is 20 or more inches.