Fetal Development Week By Week

Your Unborn Baby’s Development

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The development of the fetus inside your body is an amazingly complex process, one that starts the formation of a zygote. As tiny as a dot, the zygote gradually transforms into a fully developed baby inside the womb. Are you curious about how your baby changes and grows during each week of pregnancy?

If yes, MomJunction details the fetal development stages in each week of pregnancy.

Baby’s Development In The First Trimester

The first trimester of your pregnancy is from weeks 1 to 12. Here is how the baby develops during each week of pregnancy during the first trimester.

Size of the baby

WeekSizeLengthWeight
1 to 3Not measurable during these weeks.
4Poppy seed0.04in (0.1cm)<0.035oz (1g)
5Peppercorn0.04 in (0.1cm)<0.035oz (1g)
6Pomegranate seed0.4in (1cm)<0.035oz (1g)
7Blueberry0.4in (1cm)<0.035oz (1g)
8Cranberry bean0.62in (1.6cm) (crown to rump)0.035oz (1g)
9Cherry0.9in (0.1cm)0.07oz (2g)
10Kumquat1.22in (3.1cm)0.14oz (4g)
11Brussels sprout1.61in (4.1cm)0.25oz (7g)
12Lime2.13in (5.4cm)0.5oz (14g)

Week 1 to 3:

At week 1, you have your menstrual flow, which is followed by ovulation in the second week. If the sperm meets the egg within 12 to 24 hours of ovulation, then fertilization happens. The fertilized egg is called a zygote at this stage. It travels through the fallopian tube during the third week and multiplies into a complex cell structure called blastocyst that finally attaches to the uterine wall. Then, conception happens.

Week 4:

  • At week 4, the embryo starts producing the human chorionic gonadotropin (hCG) hormone that helps maintain the endometrium lining (1).
  • This hormone signals the ovary to stop releasing eggs for the next few months, halting the monthly periods.
  • The embryo has three layers called ectoderm, mesoderm and endoderm, which develop into the tissues and organs of the body (2).
  • Limb buds and eyes begin to appear (3).
  • Heart starts beating and the blood vessels start circulation (4).

Week 5:

  • Face attains a reptilian appearance (5).
  • Ectoderm develops into the neural tube, which further extends into the brain and spinal cord.
  • The nervous system is developing (6).
  • The gastrointestinal tract begins to form (7).
  • Arm and leg buds start to appear with webbed fingers.

[ Read: First Month Of Pregnancy ]

Week 6:

  • Hands and feet have a paddle-like look (8).
  • A portion of the brain called cerebral cortex is formed (9). Synapses, or neurons or nerve cells that communicate with each other, are formed.
  • The ears begin to take shape, and the mouth develops the salivary glands.
  • The diaphragm is formed, and a portion of the intestine begins to develop.
  • Adrenal cortex forms over each kidney, and the pancreas starts producing glucagon.

Week 7:

  • The facial features such as the eyes, ears, mouth, and nose are distinct by now (10). The umbilical cord that carries oxygen and nutrients for the baby is formed and functional.
  • The brain divides into three parts, i.e., forebrain, midbrain, and hindbrain. The brain cells are generated at a rate of 100/minute (11).
  • Skin appears to be thin and translucent.
  • The nephrons and basic filtration units of the kidneys start to form (12).
  • The intestine is developing and the digestion process begins.
  • Liver starts producing red blood cells, and the pancreas starts forming insulin.

Week 8:

  • The head of the baby is disproportionately large (13).
  • The heart develops into four chambers by now (14).
  • The spine develops the vertebral column.
  • Electrical activity starts between the brain and the nervous system.
  • The umbilical cord continues pumping blood to the embryo.
  • Eyes are developing with pigmentation of the retina. The intestine starts getting longer (15).

Week 9:

  • The eyes and mouth are distinct; the tongue develops taste buds (16).
  • Liver starts making the blood cells (13).
  • The muscles are forming, allowing slight movement of limbs.
  • Arms and elbows are developing. The nipples and hair follicles begin to form.
  • All the essential organs of the body are developing now (7).
  • Skeleton starts to form.
  • The blood vessels are visible beneath the transparent skin (17).
  • Fingers and toes are beginning to grow. (18).

[ Read: Third Month Pregnancy ]

Week 10:

  • In this week, the embryo transitions into a fetus, meaning ‘a young one’ in Latin (14).
  • The head is about half the size of the body.
  • The face attains a human-like appearance.
  • The bone cells begin to form, along with the skeletal structure (19).
  • Fingers and toes develop nails.
  • Eyelids are developed, and ears start taking shape (7).

Week 11:

  • Hands begin to open and close into fists (20).
  • The heart is completely formed and starts pumping blood (21).
  • Brain and nervous system are developing.
  • Fingernails are growing.
  • Mouth starts developing the teeth buds.
  • Intestine starts absorbing glucose and water from the swallowed amniotic fluid (22).
  • Genitals of the baby can be determined through an ultrasound scan.

Week 12:

  • Fingers and toes are distinguishable but remain webbed (13).
  • All the major body organs are formed but not yet functional fully (14).
  • Eyelids are closed and heartbeat can be detected electronically.
  • Vocal cords are formed by now (23).
  • Kidney starts producing urine.
  • The intestine makes way into the abdomen.
  • Arms are proportionate to the body size, but legs grow comparatively slowly (24).
  • The fetal colon contains meconium, which is composed of proteins, dead cells shed by the digestive tract, and the digestive enzymes.

[ Read: 3rd Week Pregnancy ]

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Baby’s Development In The Second Trimester

The pregnancy weeks from 13 to 26 mark the second trimester. During this period the baby undergoes major developments.

Size of the baby

WeekSizeLengthWeight
13Peapod2.91in (7.4cm)0.81oz (23g)
14Lemon3.42in (8.7cm)1.15oz (43g)
15Apple3.98 in (10.1 cm)2.47 oz (70 g)
16Avocado4.57in (11.6cm)3.53oz (100g)
17Turnip5.11in (13cm)4.93oz (140g)
18Bell pepper5.59in (14.2cm)6.7oz (190g)
19Tomato6.02in (15.3cm)8.47oz (240g)
20Banana10.08in (25.6cm)10.58oz (300g)
21Carrot10.51in (26.7cm)12.7oz (360g)
22Spaghetti squash10.9in (27.8cm)15.17oz (0g)
23Large mango11.38in (28.9cm)1.1lb (501g)
24Ear of a corn11.8in (30cm)1.32lb (600g)
25Rutabaga13.62in (34.6cm)1.46lb (660g)
26Scallion14.01in (35.6cm)1.68lb (760g)

Week 13:

  • Eyelids remain closed, protecting the eyes (25).
  • Lips and nose are fully formed and baby can make facial expressions (26).
  • Bones are connected by ligaments now, with the baby’s hand having 27 bones (27).
  • Fingers and toes no longer remain webbed.
  • Unique finger and toe prints are developed.
  • Hair starts growing on the head (28).
  • Placenta continues making progesterone and estrogen to maintain the pregnancy (29).

Week 14:

  • Taste buds are present all over the mouth and tongue (26).
  • Arms are long and thin and growing proportional to the size of the body (25).
  • Fine hair called lanugo is forming on the body (30).
  • The thyroid gland is matured and starts secreting the thyroid hormone.
  • Genitals are fully developed.

Week 15:

  • Teeth buds are still developing (26).
  • The fetus starts responding to light touch.
  • Skin remains transparent and soft, making the blood vessels visible (31).
  • Skeletal and muscle development continues.
  • The baby makes lots of movements with hands, feet, legs, and wrists.
  • Baby Starts curling the fingers and toes (25).
  • Starts swallowing, sucking, and breathing.

[ Read: 13th Week Of Pregnancy ]

Week 16:

  • Sweat glands start developing (5).
  • The baby can hold the head erect (32).
  • Facial muscles are forming complex expressions like frowning and squinting.
  • Sex of the baby is distinguishable in this week.
  • Legs are getting longer (14).
  • Eyebrows and eyelashes appear by now (13).

Week 17:

  • The body size is now proportional to the head (33).
  • The Skin is wrinkled and coated with vernix (34).
  • The nails grow to the tip of the fingers and toes.
  • The gallbladder starts secreting bile, which is required for the digestion process.
  • The umbilical cord is growing thicker and stronger.
  • Eggs are formed in the ovaries of the female fetus.
  • The blood cells form inside the bone marrow (26).
  • The kidney starts passing urine in every 50 minutes and is swallowed by the baby through the amniotic fluid (35).
  • The taste buds developed and can distinguish between sweet and bitter.

Week 18:

  • The middle ear bone and the nerve endings are developing to help the baby hear clearly (36).
  • The bones are hardening in the legs and the clavicles.
  • The retina can respond to light.
  • The nerves are getting covered by a tissue coating called myelin (37)
  • Formation of breathing passage called bronchial tree is complete by this week (26).

Week 19:

  • The brain is developing sensory areas for smell, vision, touch, taste, and sound (38).
  • The baby can hear loud noises.
  • The skin is covered with lanugo and a coating called vernix.
  • The baby does more active movements.
  • The face is more defined (39).
  • The female fetus contains six million eggs in the ovaries (40).

[ Read: 16th Week Pregnancy ]

Week 20:

  • Oil glands of the skin, called sebaceous glands, begin to work (14).
  • The baby’s fluttering movements called quickening can be felt in this week.
  • The fetus starts sucking the thumb, grasping and experiencing hiccups.
  • The downy hair called lanugo begins to disappear (41).
  • Permanent teeth are formed beneath the gums (42).
  • The heart is beating at the rate of 120-160 beats/minute.

Week 21:

  • Breathing pattern, body movements and the heartbeat follow a rhythmic pattern called circadian rhythms (26).
  • Liver and spleen continue to contribute to cell formation (43). Now, the bone marrow also starts to produce blood cells.
  • The baby’s movements get stronger.
  • Reflexes are developed.
  • The digestive system is functioning well (44).
  • The skin has the coating of vernix.
  • The brain is growing and its surface is smooth (45).

Week 22:

  • Lower limbs are fully developed (14).
  • Toenails are developing.
  • Meconium is made in the intestine (13).
  • Eyelashes and eyebrows appear clearly.
  • The baby’s kicks are more prominent.
  • The body is covered with lanugo.
  • Rapid eye movements begin this week (26).

Week 23:

  • Fat deposition beneath the baby’s skin begins.
  • Lungs develop the lower airways (alveoli) (5).
  • The baby is developing reflexes and can get startled by noise and light (13).
  • The skin is wrinkled (44).
  • The bone marrow is forming the blood cells.

Week 24:

  • A steady weight gain by the fetus this week (14).
  • Skin remains wrinkled and red.
  • Eyelashes and eyebrows are recognizable.
  • 30 million heartbeats occur by this week (26).
  • Eyelids are separated into upper and lower lids but remain fused shut (13).

[ Read: 26th Week Pregnancy ]

Week 25:

  • The baby breathes at a rate of 44 times/ minute (26).
  • The alveoli create surfactant that maintains the surface tension inside the lungs (5).
  • The inner ear is completely developed (45).
  • Fat is depositing under the skin.
  • The baby follows a sleep and awake pattern (44).

Week 26:

  • Eyes begin to open and blink (46).
  • Eyelashes are growing and there is more hair on the head.
  • Footprints and fingerprints are formed.
  • Lungs are still developing and not fully mature (7).
  • The baby’s heart rate and movement increase as the baby may respond to sudden noises or movement (26).

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Baby Development In The Third trimester

Weeks 27 to 40 of pregnancy form the third trimester. During this period, the baby is still developing and by the end of the trimester, he/she is ready to enter the new world.

Size of the baby

WeekSizeLengthWeight
27Cauliflower14.41in (36.6cm)1.93lb (875g)
28Eggplant14.8in (37.6cm)2.21lb (1,005g)
29Butternut squash15.19in (38.6cm)2.54lb (1153g)
30Cabbage15.71in (39.9cm)2.91lb (1319g)
31Coconut16.18in (41.1cm)3.3 lb (1502g)
32Jicama16.69in (42.4cm)3.75lb (1702g)
33Pineapple17.2in (43.7cm)4.23lb (1918g)
34Cantaloupe melon17.71in (45cm)4.73lb (2146g)
35Honeydew melon18.19in (46.2cm)5.25lb (2383g)
36Romaine lettuce18.66in (47.4cm)5.78lb (2622g)
37Swiss chard19.13in (48.6cm)5.25lb (2383g)
38Leek19. 61in (49.8cm)6.8lb (3083g)
39Mini watermelon19.96in (50.7cm)7.25lb (3288g)
40Small pumpkin20.16in (51.2cm)7.63lb (3462g)

Week 27:

  • Lungs, liver and the immune system are developing (47).
  • The skin has the coating of vernix.
  • Auditory systems continue to develop but the baby may begin to recognize your voice or your partner’s.
  • The retina is developed and the baby may be able to distinguish between light and dark (5).
  • Thigh bones and foot bones measure 2in (5cm) long (26).

Week 28:

  • The folds and grooves in the brain are still developing (48).
  • Fat, which maintains the body temperature, continues to accumulate beneath the skin.
  • The nervous system is developed and controls a few bodily functions (7).
  • Eyes start producing tears and nose can smell (26).

[ Read: 31st Week Pregnancy ]

Week 29:

  • The baby continues to be active with a lot of movements.
  • The lungs are capable of breathing (49).
  • The bone marrow is forming the red blood cells.
  • The eyes begin to open and close frequently.
  • The brainwaves emitted indicate the consciousness and pain (5).
  • The eyes react to light (26).

Week 30:

  • The baby looks plump due to fat deposition under the skin (50).
  • The skin is less wrinkled.
  • The baby sleeps for a long time (5).
  • The brain is still developing (44).
  • The lungs are getting matured (26).

Week 31:

  • Urination frequency of the baby increases (51).
  • In boys, testicles make way to the scrotum (5).
  • The baby’s body starts absorbing minerals like calcium and iron from the intestine.
  • 40 million heartbeats have occurred by this time (26).
  • Wrinkles start disappearing with the fat deposition under the skin.
  • Bones are fully developed but are soft, and lungs continue breathing (7).

Week 32:

  • Lanugo begins to fall off, but some may remain on the shoulder and the back (52).
  • Kidneys are fully developed (5).
  • The baby moves into the head down position.
  • Breathing movements are occurring.
  • Kicks and movements are stronger than before.
  • The baby spends most time sleeping.

[ Read: 39th Week Pregnancy ]

Week 33:

  • Billions of neurons are still developing in the brain (53).
  • The pupil in the eyes can constrict and dilate as a response to light.
  • Rapid eye movement is occurring now.
  • Fat deposition still continues.
  • Lungs are almost formed, but not fully matured.

Week 34:

  • Lanugo is almost disappeared, but the vernix coating will still be there (5).
  • The baby attains the right birthing position.
  • Fewer movements happen due to less room inside the womb.
  • The true alveoli or ‘air pockets’ in the lungs are still developing (26).
  • Fingernails have reached the fingertips (14).
  • Skin is pink and smooth (26).
  • Kicks are much stronger.

Week 35:

  • Vernix coating is becoming thicker (54).
  • Lanugo is almost off.
  • The lungs are creating surfactant (5).
  • The bones and muscles are fully developed. Bones are still soft.
  • The movements are more restricted.
  • The baby moves into the pelvis with a head down position.

Week 36:

  • Skull bones are soft and can overlap as they move through the birthing canal (55).
  • The body is covered with a layer of fats.
  • The limbs are fully formed with fingernails and toenails (14).
  • Muscle tone is developed. It helps the fetus to lift and turn the head.
  • Earlobes and cartilage are soft.
  • Blood vessels are completely developed.

Week 37:

  • The fetus develops firm hand grip (26).
  • By this week, the heart has beaten more than 50 million times.
  • The baby has definite sleep patterns.
  • Fat is deposited all over the body.
  • Movements are less and the baby is positioned deep into the pelvis, in the head-down position.

[ Read: 41st Week Pregnancy ]

Week 38:

  • Pregnancy is considered full term at this week.
  • Fat is still forming and the fetal growth is almost stopped (56).
  • Most of the lanugo and vernix is gone (5).
  • Facial skin becomes smooth (14).
  • Head is the largest part of the body now.
  • Small breast buds are present in both the sexes (7).
  • Head has coarse and thicker hair.

Week 39:

  • The baby is considered full term by this week.
  • The umbilical cord measures about 20 to 24 inches in length.
  • Most of the vernix is shed off.
  • The placenta is supplying oxygen, nutrients, and antibodies to the baby (57).

Week 40:

The baby is fully developed and has attained the growth of 38 weeks since the day of conception. He/she is ready to be born any day now.

The fetal development during each week signifies the developmental milestone of your baby. After a long wait, your baby finally arrives, bringing all the joy you have been anticipating. It is now your turn as parents to start taking up the responsibilities of the baby’s well-being.

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Have any experiences to share about your pregnancy? Let us know in the comment section below.

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shreeja pillai

Postgraduate in Chemistry and content writer. She has worked as a research analyst with a leading multinational pharmaceutical company and also holds a diploma in pharmaceutical regulatory affairs. Her interest in the field of medical research has developed her passion for writing research-based articles. She is a writer for MomJunction and aims at providing informative articles based on health and wellness. Apart from writing, she takes a great interest in music and traveling.
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