From Conception to Birth
How a baby develops during pregnancy
The First Trimester (Fertilisation - 12 weeks)
At the time of sexual intercourse the egg or ovum is a single cell and is just 0.13mm (0.005in) in diameter. When the ovum is fertilized by a sperm cell (up to 72 hours later) their genetic material combines. The egg is now referred to as a zygote as the cells split in a process known as cleavage and the start of embryonic development.
30 hours after fertilization the egg divides into two identical cells. Around 10 hours later these two cells divide again making 4 cells.
Within 3 days of fertilization the egg has divided into 16 cells. Whilst dividing the fertilized egg moves along the c towards the womb.
The fertilized egg is now a mass of about 64 cells and passes out of the fallopian and into the womb.
The cell mass fills with fluid secreted by glands in the lining creating a central cavity by displacing a cluster of cells to one side. This cell cluster develops into the embryo. The remaining cells which surround them will begin to burrow into the lining of the womb.
The developing embryo settles into the folds in the lining of the womb. During the two weeks prior to conception the womb lining has become engorged with blood ready to receive and nourish an embryo.
The embryo begins to burrow into the lining of the womb, a process known as implantation, in search of protection and nourishment.
The outer layer of the trophoblast (the cells which surround the embryo) divide into two layers. The outer layer burrows deeper into the lining of the womb, while the inner layer begins to lay the foundations of the placenta, the structure through which the embryo will receive nutrients from the mother's blood.
The inner cells of embryo divide into two layers - the ectoderm and the endoderm -from which the tissues and organs of the body will eventually develop.
The amniotic sac which will soon form a protective fluid filled 'balloon' around the embryo starts to develop.
The placenta begins to develop as the outer cells of the embryo send finger-like projections, called villi, into the surrounding tissue.
The embryo, which is now a disc-shaped mass of cells 0.2mm in diameter, is completely embedded in the womb.
At the beginning of the 3rd week a groove, called the primitive streak, forms toward what will be the tail end of the embryo. A new layer of tissue, called the mesoderm, develops from the primitive streak. The spine, the kidneys and some of the major tissues of the body eventually develop from the mesodermal tissue.
Cells from the ectodermal tissue within the embryo move downward to create the neural fold and upward to create the neural plate . These creations are the first stages in the development of the nervous system.
At the end of the third week the neural fold moves upward and forms the structure called the neural groove. This will become the spinal cord. At the same time blocks of tissue called somites form from the mesodermal tissue on the other side of the neural groove. The vertebrae (the bones of the backbone) the ribs and muscles of the thorax develop from the somites.
A tube-like heart has developed. By the 24th or 25th day it will begin to beat.
The kidneys are forming from mesodermal tissue. The mouth begins to develop.
The embryo is now 3mm (0.1in) long and roughly cylindrical in shape. It is being supplied with oxygen and nutrients through the placenta, although it is not fully developed. A rudimentary spinal cord and gut run from the head to the tail.
Since the tissues that make up the back of the embryo develop more rapidly than from the front of the body, the head and tail fold downward so that the embryo begins to grow into a curve.
The heart tube bends into a U-shape and blood begins to circulate around the body.
The embryo is now 5mm (0.2in) long. The connecting stalk by which the embryo has been attached to the placenta begins to grow into an umbilical cord. By the 12th week it will contain 2 arteries which carry blood to the placenta and a vein through which the blood is returned to the fetus. The tongue begins to form in the embryo's mouth.
The arms and legs begin to develop as the paddle-shaped buds. Pharyngeal arches which develop in the face, jaws, throat and neck appear between the head and body.
A complex network of nerves and blood vessels is developing throughout the embryo's body.
The embryo's eyes have formed and the lenses have become visible. The ears which have been developing inside the head for some days have just become visible.
The spleen (an organ which manufactures red blood cells) and the pancreas (an organ which secretes digestive juices and hormones) start to develop in the central part of the gut.
The thymus and the parathyroid glands develop from the third pharyngeal arch. The thymus gland produces white blood cells which play an important role in the body's defences against disease and infection, the parathyroid controls the amount of calcium in the blood.
Primordial germ cells (the cells that become the future sperm and oocytes or eggs) form in the developing sex glands.
The ends of the limb buds have flattened into plate-like structures from which the hands and feet develop during the next few days.
The length of the embryo from the crown of the head to rump is 7-8mm (0.4in) long. By the end of the 5th week the full complement of 42 tissue blocks, or somites, has formed along the embryo's back and the development of the backbone, ribs and muscles of the torso now begins.
The length of the embryo from the crown of the head to rump is 12.5mm (0.5in) long. By the 6th week after conception the embryo's heart has established a regular rhythm and the stomach has formed. The ears, nose, fingers and toes are now forming.
The eyelids form from a single membrane which remains fused for some days. The limb muscles are beginning to form. The chest cavity becomes separate from the abdominal cavity by a band of muscle. This later develops into the diagram , a muscle which plays an important role in breathing.
Between the 4th and 8th weeks the brain has been developing so rapidly that the head is huge in proportion to the rest of the body. The gonads or sex glands start to develop into ovaries or testes between the 7th and 9th weeks. The elbow and fingers, knees and toes are taking shape. The lungs are developing inside the chest cavity.
The developing baby is called an embryo only up to the end of the 8th week. From the beginning of the 9th week it is called a fetus. By now the face is more human. The back of the head has enlarged so that the eyes seems to be in a more normal position. The ears look flatter and the nose has formed. The limbs have grown more in proportion with the rest of the body and the anus has developed.
Inside Pregnancy: Weeks 10 - 14 (BabyCenter Video)
The neck develops between the head and chest. Nails grow on the fingers and toes, the fingerprints are also forming. Blood begins to surge through the lungs at a rate of about 80 times a minute.
The length of the fetus from the crown of the head to rump is 55mm (2.2in). The primitive gut completes its transformation into the alimentary canal, the passage along which food travels from the mouth to the anus. The kidneys begin to make a little urine which trickles into the bladder. The limb muscles start to move, but their contractions cannot yet be felt by the mother.
The fetal muscles begin to make reflex movements ie the hands make grasping movements. This is a sign that the nervous system has begun to function. Red blood cells which were formerly made in the liver, the spleen and in the primitive yolk sac which formed at the embryo stage now begin to be made by the bone marrow.
Face Development in the Womb - Inside the Human Body: Creation - BBC One
The Second Trimester (13 - 24 weeks)
The 3 tiny bones of the middle ear, the malleus, incus and stapes are the first bones to begin to harden making the fetus capable of hearing. The skeletal bones gradually harden as bone tissue replaces the cartilage from which the fetal skeleton first develops.
The external genital organs have now developed enough for the fetus's sex to be detectable by ultrasound. The skin of the fetus is developing. At this stage it is transparent but looks red because the blood vessels can be seen through it. The muscles of the chest start to make movements similar to those of respiration.
Inside Pregnancy: Weeks 15 - 20 (BabyCenter Video)
The skin and muscles are now fully formed. All the joints are able to move.
The length of the fetus from the crown of the head to rump is 110mm (4.4in). The nasal septum, the cartilage that separates the two nostrils, fuses together with the palate in the roof of the mouth. Deep inside the developing lungs tiny air sacs called alveoli are starting to form.
The start of the 5th month the fetus begins to swallow. This causes it to drink amniotic fluid at the rate of about 400ml a day (about half of the amount within the amniotic sac). At the same time the gut begins to secrete gastric juices which help in the absorption of the fluid. The fluid is filtered by the kidneys and excreted into the amniotic sac. the brain tissues begins to form folds at the cortex (or grey matter) outgrows the white matter beneath it.
The skin becomes opaque and the first coat of fine hair known as lanugo begins to grow on the surface. White blood cells or leucocytes are being manufactured. They play an important part in fighting disease and infection.
The tongue is now fully developed. the internal organs of reproduction the vagina and the womb have formed in the female fetus.
By the end of the 5th month the fetus has grown to a length of about 160mm (6.5in). Its movements as it turns in the womb and moves its limbs and sucks thumb, can be felt as 'quickening' by the mother. Vernox caseosa a whitish fatty substance is beginning to form on the fetus's skin. It will lubricate the baby's passage along the birth canal and it's believed also to protect the delicate newly formed skin from the increasing concentration of urine in the amniotic fluid.
Inside Pregnancy: Weeks 21-27 (BabyCenter Video)
The tonsils have developed in the throat. The tonsils, like the thymus gland manufacture white blood cells or leucocytes which help fight infection. The pancreas a gland behind the stomach begins to secrete insulin a hormone which controls the level of sugar in the blood.
The fetus weighs about 250gm (1/ lb). The eyebrows grow and the hair on the head begins to thicken.
The fetus looks wrinkled because, although it is almost fully developed, very little fat has as yet been deposited beneath the skin.
The fetus measures from the crown of the head to rump is 200mm (8in). The branches of the lungs, called bronchi, are developing but the lungs will not be fully formed until after birth.
The Third Trimester (25 weeks - Birth)
The eyes open as the membrane from which the eyelids formed parts.
The fetus is now turning into different positions in the womb and moving its limbs. It can cough and hiccup.
The nerve cells in the brain are becoming more specialised and as a result the cerebral cortex (the outer layer of the brain which is responsible for intellect, perception and other aspects of consciousness) is becoming organised into the different areas responsible for sensation and other specialised activities.
Inside Pregnancy: Weeks 28 - 37 (BabyCenter Video)
The fetus measures from the crown of the head to rump is 240mm (9.5in) and weighs about 1 kg (2 lb). The pupils of the eyes are now capable of dilating and contracting in response to the presence or absence of light. Before the 28th week the death of an embryo or fetus is termed a miscarriage and does not have to be registered with the authorities. From the 28th week, fetal death is termed stillbirth and must be registered by law with the register of Births, Deaths and Marriages.
By the end of the 7th month in the womb the air sacs in the lungs have been lined with a layer of cells which produce a liquid called surfactant. This prevents the air sacs from collapsing when the baby begins to breathe after birth.
At the beginning of the 8th month the fetus can blink. The development of the ear is completed by the formation of the eardrum.
The body of the fetus has grown so that it is now in proportion to the head. If the baby were to be born around this time it would have a good chance of survival.
The fetus measures from the crown of the head to rump is 280mm (11in). In the male fetus the testes have passed down the inguinal canal and into the scrotum by week 32.
Between week 33 and birth, some 870g (2lb) of fat is deposited all over the body but principally beneath the skin and around the shoulders so that the fetus loses its wrinkled appearance and becomes well rounded.
By the end of the 3rd trimester the fetus measures from the crown of the head to rump is 310mm (12.5in). The brain grows rapidly between week 34 and birth. Sleep rhythms are detectable on an electro-encelphalogram (EEG) a device that records brain activity.
The fine lanugo hair starts to disappear from the body except from an area around the shoulders and the body creases. During the 3 weeks before delivery the fetus usually turns into a longitudinal head-down position and the head descends into the lower part of the uterus ready for birth. This is called lightening.
The eyes of the fetus are usually a dull blue colour since pigment does not form in the iris until after birth.
At birth the fetus has smooth skin and is covered in vernix. The nails have grown to the ends of the finger and toes. The fetal rate is about 140 beats per minute. The baby's first breath leads to major changes in the heart, blood pressure and the circulation. After delivery the baby usually suckles at the mothers breast and may sleep for 24 hours.
The Social Baby Video in partnership with NSPCC
The Social Baby Book
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A touch-and-feel board book for your baby to explore (First Baby Days)
A two-sided developmental toy. One side a mirror, the other developmentally appropriate black and white graphics.