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Week 3 |
Fertilization/conception
Gestational age: 2+0
Conceptional age: Week one, 0+0 - 0+6, the first to the seventh day after conception
A spermatozoon (sperm cell with genes from the father) fuses with an oocyte (the ovum with genes from the mother) forming a zygote.
Now a process called genetic recombination takes place. Gene sequences from the mother's and father's chromosomes are replaced with each other, forming an entirely new human being with its own unique set of genes!
The genes contain a coded instruction for what is going to happen in the future development of the child.
The picture shows a sperm cell entering the ovum.
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Week 3 |
Just a lump of cells?
1st day: fertilization
2nd day: two-cell stage (see photo), divides further into 8 cells, forming a blastomer
3rd day: the child now consists of 16 cells which are called the morula: it is not a lump of non-differentiable cells, but shows a clear subdivision of inner cells (the embryo proper: the child), and outer cells (the trophoblast: becomes the placenta and fetal membranes). Every cell has strict orders of the next step!
4th day: early blastocyst
5th day: late blastocyst
6th day: implantation in the uterine mucosa (embedding in the uterine lining)
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Week 4 |
Gestational age: 3+0 - 3+6
Conceptional age: Week two, 1+0 - 1+6, 8-14 days after conception
Length: CRL 0.1 mm
Milestones:
Formation of the so-called bilaminar disc: The child now consists of two cell layers.
Other cells develop into the placenta.
The foundations for many organs are laid: nervous system, gastrointestinal tract, lungs, cardiovascular system, skin and muscles.
Image: UNSW Embryology 2007
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Week 5 |
Gestational age: 4+0 - 4+6
Conceptional age: Week three, 2+0 - 2+6, 15-21 days since conception
Length: CRL 0.2 to 3.0 mm
Milestones:
The child is now called an embryo, and consists of three layers of cells, called the trilaminar disc.
The top layer is called the ectoderm, from which the outer layer of the skin, central nervous system (brain and spinal cord), peripheral nervous system (nerves), eyes, inner ear and connective tissues are derived.
The middle layer is called the mesoderm. From here the heart, blood vessels with primitive blood cells, bones, muscles, kidneys and much of the sexual organs form.
The innermost layer of cells, the endoderm, becomes a long tube lined with mucous membranes. They evolve further to become the lungs, intestines and bladder.
Every cell knows what to do, and migrates to its proper place from all the three layers as the child grows.
The central nervous system develops further to e.g. the cerebral hemispheres (the brain's two large halves) and the medulla spinalis (the spinal cord).
After the formation of the cardiovascular system (the heart and blood vessels), the uteroplacental circulation begins (the exchange of oxygen and nutrients between the uterus and the placenta).
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Week 6 |
Gestational age: 5+0 - 5+6
Conceptional Age: Week four, 22-28 days after conception
Length: CRL 3.0 to 5.0 mm
Weight: 1 gram
Milestones:
Rupture of the buccopharyngeal membrane (the mouth opens)
Development of the branchial arches
Formation of the heart, which begins to beat (the heart is the red sphere in the middle of the photo)
24-25 days: the eyes and ears are being formed with creation of the optic vesicles and otic placodes
26-27 days: arm buds appear
28-30 days: leg buds appear, otic vesicles, lens placodes
The baby's first movement: bending of the head and neck
(Wikipedia: Fetal movement)
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Week 7 |
Gestational age: 6+0 - 6+6
Conceptional age: Week five, 4+0 - 4+6, 29-35 days after conception
Size: CRL 5-9 mm
Milestones:
The face takes shape: the nostrils and the jaw develop
the extremities are subdivided: the hands, feet, elbows and knees appear.
Movement of the arms and legs can now be observed with an ultrasound scan
(Wikipedia: Fetal movement)
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Week 8 |
Gestational age: 7+0 - 7+6
Conceptional age: Week six, 5+0 - 5+6, 36-42 days after conception
Length: CRL 10-14 mm
Milestones:
More contractures on the hand and foot shape the fingers and toes.
Formation of the brain vesicles and the extern auricle (the outer ear)
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Week 9 |
Gestational age: 8+0 - 8+6
Conceptional age: Week seven, 6+0 - 6+6, 43-49 days after conception
Length: CRL 15-22 mm
Weight: 3.4 grams
Milestones:
The cloacal membrane degrades and the anus opens.
Visible pigmentation of the retina.
Formation of the eyelids, upper lip and nipples.
Free Abortion up to 10 weeks:
Slovenia, Portugal
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Week 10 |
Gestational age: 9+0 - 9+6
Conceptional age: Week eight, 7+0 - 7+6, 50-56 days after conception
Length: CRL 23-31 mm
Weight: 5 grams
Milestones:
More contractures creates elbows and knees.
Free fingers and toes.
The tail disappears.
Genital organs are formed, but it is yet too early to determine the gender.
The face is further developed and the eyelids start to close to protect the evolving eyes.
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Week 11 |
Gestational age: 10+0 - 10+6
Conceptional age: Week nine, 8+0 - 8+6, 57-63 days after conception
Length: CRL 5 cm
Weight: 10 grams
Milestones:
The child is now called a fetus.
During the third month the eyes and ears move to their proper places, and the limbs reach their relative length.
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Week 12 |
Gestational age: 11+0 - 11+6
Conceptional age: Week ten, 9+0 - 9+6, 64-70 days after conception
Length: CRL 6 cm
Weight: 20 grams
Milestones:
Fingernails develop.
Primary ossification centres (incipient bone cores) in the skull and long tubular bones (eg. femur).
The sex can know be determined.
Abortion until 12 weeks:
Denmark, Germany, France, Italy, Greece, Bulgaria, Hungary, Slovakia, Lithuania, Latvia, Estonia, Austria
Abortion up to 12 weeks if the pregnancy harms the mother's physical or mental health:
Poland
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Week 14 |
The blood vessels easily show through the thin skin, due to the lack of subcutaneous fat.
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Week 14 |
Gestational age: 13+0 - 13+6
Conceptional age: 3 months, 11+0 - 11+6, 12 weeks since conception
Length: 8 cm CRL
Weight: 45 grams
Abortion until the 14th week:
Slovenia
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Week 16 |
Gestational age: 15+0 - 15+6
Conceptional age: 3 1 / 2 month, 14 weeks since conception
Length: 10 cm CRL
Weight: 80 grams
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Week 18 |
Gestational age: 17+0 - 17+6
Conceptional age: 4 months, 16 weeks since conception
Length: 14 cm CRL
Weight: 200 grams
Milestones:
4-5. month: The child grows in length and is covered with lanugo hairs. At the same time the eyebrows and scalp hair appear.
It is during this period the mother begins to feel the child's movements inside her stomach.
Abortion until the 18th week:
Sweden
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Week 22 |
Gestational age: 21+0 - 21+6
Conceptional age: 5 months, 20 weeks since conception
Length: 19 cm CRL
Weight: 450 grams
Abortion if the pregnancy harms the mother's physical or mental health:
Spain
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Week 24 |
Gestational age: 23+0 - 23+6, 6 months
Conceptional age: 5 months, 21 weeks since conception
Length: 21 cm CRL
Weight: 600 grams
Milestones:
Pay special attention to the baby's fine lanugo hairs.
The skin is reddish and wrinkled, as there is no underlying connective tissue.
Abortion if the pregnancy harms the mother's physical or mental health, or she is in a bad economic or social situation:
England, Finland
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Week 26 |
Gestational age: 25+0 - 25+6
Conceptional age: 6 months, 24 weeks since conception
Length: 23 cm CRL
Weight: 820 grams
A baby can be born from 22 weeks of age, but has difficulty surviving before the 28th week due to lack of differentiation within and between the central nervous system and the respiratory system.
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Week 32 |
Gestational age: 31+0 - 31+6
Conceptional age: 7 1/2 months, 30 weeks since conception
Length: 29 cm CRL
Weight: 1,600 grams
Milestones:
During 8-9 months of age, there is considerable weight gain, and the contours become rounded due to fat deposits.
Sebaceous glands secrete a whitish, greasy substance, the vernix caseosa, which covers the child.
The testes (the testicles) have now dropped down into the scrotum (the groin).
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Week 40 |
Gestational age: 39+0 - 39+6
Conceptional age: 9 months, 38 weeks since conception
Length: CRL 36 cm, head-heel: 50 cm
Weight: 3,400 grams
Birth!
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