Embryonic development in the first 8 weeks of pregnancy
During pregnancy we speak of embryonic development up to the 10 completed weeks of gestation (counting from the date of the last menstruation). Fetal development starts from the eleventh week (therefore from 10+0).
When a woman takes a pregnancy test and gets a positive, the embryo has already been developing for several days.
And those days are of paramount importance for the success of the pregnancy and for the health of the baby.
In the first 10 weeks of pregnancy, which are actually 8 (or 56 days) of life of the embryo if we consider ovulation which took place two weeks after the last menstrual period, an extraordinary series of events take place which lead a single fertilized cell to become a fetus of about 27-31mm in length.
We will go here to see, in detail, step by step, and week by week of pregnancy, the development of the embryo. This is a very delicate and fundamental phase also for the future health of the child. For this reason it’s important that mom and dad take care of him already in these very early stages of pregnancy, when it’s not yet known if the woman is pregnant, because it’s not yet time to take a test. Both must avoid cigarette smoke (passive smoke is also harmful) and for the mother alcohol, drugs, medicines, infected foods ( toxoplasmosis )…
Fundamental in this phase the intake of folic acid , which should be taken from the moment you decide to have a baby, to arrive at this stage with stocks already present, in order to reduce the risk of malformations such as spina bifida.
The development of the embryo in the first 56 days
The first 56 days of life of the embryo are divided into 23 stages of development called Carnegie stages.
The stages have been defined according to the development of various embryonic structures (from the works of Streeter (1942), O’Rahilly and Müller (1987)) starting from the first day after conception.
As we know, the first two weeks of pregnancy cover the period from the last menstrual period to the day of ovulation (assuming ovulation on the 14th day) and in this phase the woman is not pregnant.
If conception occurs after ovulation (the oocyte remains fertilizable at most 12-24 hours after ovulation) then the actual pregnancy begins.
For simplicity in the following stages, we will consider a standard pregnancy with conception on the 14th day from the start of menstruation.
Stage 1 of embryonic development
We are in the first day after ovulation in what we call the third week of pregnancy (ranging from 2+0 to 2+6)
Conception has occurred and the precocious zygote looks like this:
As you can see, this fertilized cell has two pronuclei (one from the mother and one from the father) in the center of the cytoplasm.
At this stage the zygote is in the fallopian tube, en route to the uterus.
Stage 2 of embryonic development
We are in the third week of pregnancy, between the second and third day after conception.
The first mitotic cell divisions of the zygote initially form 2 blastomeres (blastomeres are the first cells that form from the first cell divisions) which in turn continue to divide to form the morula, so called because it looks like a berry / blackberry .
Stage 3 of embryonic development
We are in the third week of pregnancy 4-5 days after conception.
The blastocyst has formed , it is a stage that has a well-defined appearance: in fact, an internal cavity has formed (the blastocoel) visible under the microscope, which distinguishes both the trophoblast (from which the placenta and other embryonic appendages will originate) ) and the embryoblast (from which the embryo will originate).
Stage 4 of embryonic development
With its exit from the zona pellucida (a process called blastocyst hatching), the blastocyst is ready for implantation into the lining of the uterus.
Implantation can only occur during the endometrial window of receptivity which typically lasts 2-4 days beginning approximately 6 days after the start of the progesterone surge.
In this phase the trophoblast proliferates and differentiates into two cell layers: a deep one (cytotrophoblast) and a superficial one (syncytiotrophoblast).
Cytotrophoblast is the inner cell layer of the trophoblast. Its rapidly proliferating cells are obtained by chorionic villus sampling and used for karyotype and DNA analysis.
The syncytiotrophoblast is formed by cells whose activity serves to destroy the cells of the endometrium to make the blastocyst penetrate the lining of the uterus and at the same time release nutrients. It is the layer of cells responsible for the decidual reaction and production of beta HCG . The beta HCG in turn causes the corpus luteum to transform into the corpus luteum of pregnancy which will continue to produce progesterone which is essential for the maintenance of the pregnancy.
Stage 5 of embryonic development
We are 7-8 days after conception, and in the fourth week of pregnancy.
The implant process proceeds. Syncytiotrophoblast and cytotrophoblast cells continue to proliferate. Syncytiotrophoblast cells continue to invade the maternal endometrium, and cytotrophoblast cells form clusters that will later be the sites of chorionic villus formation.
The maternal endometrium begins the decidual process. The extra-embryonic cavities begin to form.
The epiblast (which will give rise to the ectoderm) and the hypoblast (which will give rise to the entoderm) are defined in the inner cell mass.
Stage 6 of embryonic development
We are about 14 days after conception and therefore at the end of the fourth week of pregnancy (more or less around 3+6).
Many women in this stage have not yet taken a pregnancy test but the embryo at this stage is beginning to take shape.
The bilaminar embryo (epiblast and hypoblast) measures approximately 0.2 mm in diameter. There are three extra embryonic spaces (amniotic, primitive yolk sac and chorionic) as you can see in the figure:The large chorionic cavity is surrounded by the cell layers which extend to form chorionic villi. These villi are the beginning of the functional units of the placenta.
Outside these villi, extending from their tips, trophoblasts invade the maternal decidua forming maternal blood lacunae (lakes). In these maternal lacunae both the uterine glands and the spiral arteries held open by the trophoblast cells afferent.
The maternal uterine wall continues to undergo decidualization in response to the implantation process.
Stage 7 of embryonic development
We are in the fifth week of pregnancy , about 15-17 days after conception.
The embryo is now 0.4 mm in diameter.
We are in full gastrulation process. During gastrulation the cells of the embryo change their localization and establish contacts with new cells. In this way signals are exchanged between cellular groups and the three-layered structural plan of the organism is formed: ectoderm, mesoderm, endoderm .
The mesoderma is found between the ectoderma and the endoderma.
- From the ectoderm will derive the nervous tissue, the epidermis, the sense organs.
- From the mesoderm will derive: bone tissue, cartilaginous tissue, connective tissues, smooth and striated muscles, blood and lymphatic vessels, kidneys, adrenal cortex, gonads and genital tracts, lympho- and hematopoietic tissues , the serous membranes,
- From the endoderm will derive the lining and glandular epithelia of the digestive tract, respiratory tract and pulmonary alveoli, tympanic cavity and auditory tube, urinary bladder and urethra, glandular epithelia of the thyroid, parathyroids, liver , the pancreas and its excretory ducts, the biliary tract, the epithelial structures of the thymus and tonsils.
At the end of the gastrulation process, the embryo will be ready for the formation of the first organs.
Gastrulation continues as cells migrate from the epiblast, continuing to form the mesoderm.
The mesoderm lies between the ectoderm and the endoderm as a continuous sheet except for the buccopharyngeal and cloacal membranes. These membranes have only ectoderm and endoderm and are found on the rostral (head) and caudal (tail) portions of the gastrointestinal tract.
From the primitive node a tube extends under the ectoderm in the opposite direction to the primitive streak. This tube first forms the axial process then the notochord process, then finally the notochord (from which the vertebral column will develop) .
The notochord is critical for embryonic folding and regulation of differentiation of ectoderm and mesoderm.
Stage 8 of embryonic development
We are 17-19 days after conception, at the stage where most expectant mothers start taking pregnancy tests.
The embryo measures 1-1.5 mm.
Distinctive signs of this stage:
- Pear-shaped embryo
- Axial process
- neural channel
- Neural sulcus formation
Stage 9 of embryonic development
We are in the fifth week of pregnancy between 19 and 21 days after conception.
The embryo is between 1.5 and 2.5 mm in size
Distinctive signs:
- Embryone with somites (1-3 somites)
- Pharyngeal membrane
- Neural sulcus
- Caudal eminence
- intraembryonic coelom
- Heart sketch, transverse septum
- Thickenings in the cephalic area after fusion of the neural crests: they represent the outline of sense organs and auditory pits
Stage 10 of embryonic development
We are 22-23 days from conception and here in the sixth week of pregnancy (ranging from 5+0 to 5+6). The embryo is about 3.5 mm long.
4 – 12 somites are counted, neuropore seen in the tail and head, neural folds seen in the developing brain region, neural tube, somites, and remnant yolk sac.
Distinctive signs:
- Lateral bending
- Closure of the neural groove in a neural tube
- The first 2 pharyngeal arches appear
- Ophthalmic vesicle
- Rupture of the pharyngeal membrane
Stage 11 of embryonic development
We are 23-26 days from conception.
The embryo is about 2.5-4.5 mm long.
The neural tube continues to close, the rostral neuropore closes.
It is a key stage in heart tube formation and neural plate development.
They characterize this stage:
- Closure of the neuropore in the tail
- Nasal thickening
- Ophthalmic vesicle
- Auditory pit
- 13 – 20 somiti
Stage 12 of embryonic development
We are 26 – 30 days from conception, in the sixth week of pregnancy. The embryo is 3 – 5 mm long.
The neural tube continues to close, the caudal neuropore closes, the forebrain is seen.
There are 21 to 29 pairs of somites, and the prominence of the heart is evident.
At the level of the head, the 1st, 2nd and 3rd pharyngeal arches, forebrain, site of the optic placode, stomodeum (initial tract of the digestive system) are seen. The heart,
liver, upper limb bosses are evident.
Distinctive signs of this stage:
- Closure of the caudal neuropore
- Third pharyngeal arch
- Sketches of the upper extremities
- Cervical sinus
- 21-29 somiti
Stage 13 of embryonic development
We are about 28-32 days (4 weeks) from conception, we are now towards the end of the sixth week of pregnancy and the embryo has a head-tail length of 4-6mm.
Distinctive signs of this stage:
- Sketches of the lower limbs
- Thickening of the lens
- Auditory pits
- Fourth pharyngeal arch
- More than 30 topics
Stage 14 of embryonic development
We have entered the seventh week of pregnancy and about 31-35 days after conception.
The embryo is about 5-7 mm long.
Distinctive signs of this stage:
- Ophthalmic vesicle
- Absence of the endolymphatic channel
- Increased cephalo-cervical flexibility
- Primarily the cerebellum
- First visible to the eye
- Sixth pharyngeal arch
Stage 15 of embryonic development
We are always in the seventh week of pregnancy at 35 – 38 days after conception.
The embryo is now about 7-9 mm long.
About 33 days after conception, the lens is covered by the ectoderm.
Outlines of the external ear and two oval depressions representing nasal cavities appear.
In the draft of the upper limbs, a distal and a proximal part is visible.
In this phase the heartbeat is present (usually 90-110 beats per minute).
Stage 16 of embryonic development
We are in the eighth week of pregnancy starting from the last menses (from 7+0 to 7+6), 37 – 42 days after conception and the embryo measures 8 – 11 mm.
The telencephalon, diencephalon, midbrain, metencephalon, myelencephalon, and spinal cord are distinguished.
- Deep nasal cavity, level with the belly
- Eye pigmentation
- Maxillary and mandibular components of the 1st pharyngeal arch
- Foot sketches
- Physiological umbilical hernia
- Nasolacrimal groove
Stage 17 of embryonic development
We are in the eighth week of pregnancy, 42 – 44 days after conception and the embryo is 11 – 14 mm long.
Characteristic signs:
- External ear canal
- Increased head circumference
- Trunk lengthening
- Fusion of the 6 auricular hillocks
- Embryonic tail atrophy
- Formation of the interdigital zones of the hands
Stage 18 of embryonic development
We are in the ninth week of pregnancy , 44-48 days after conception. The embryo is 13-17 mm long
Characteristic signs of this stage:
- Sketches of toes
- Nose sketch
- The elbow joint is formed
- In male embryos, the gender of the gonads can be determined
Stage 19 of embryonic development
We are in the ninth week of pregnancy at 48 – 51 days from conception. The embryo is 16 – 18 mm long.
Characteristic signs of this stage:
- Stretching and ironing of the bust
- external auditory canal
- Beginning of training of the hip and knee joints
- Embryonic tail atrophy
- Rupture of the cloacal membrane
- Maxilla and mandible
- Fingers
- Toes in formation
Stage 20 of embryonic development
We are in the tenth week of pregnancy which goes from 9+0 to 9+6. It is the last week in which we talk about embryonic development, from the next we will talk about fetal development.
51 – 53 days have passed since conception, the embryo is 18 – 22 mm long.
Characteristic signs of this stage:
- Fingers apart
- Gonadal gender determinable in female embryos
- Hint of pronation in the limbs
Stage 21 of embryonic development
We are in the tenth week of gestation, 53 – 54 days after conception. The embryo is 22 – 24 mm long
Characteristic signs of this stage:
- At the level of the head, the network of subcutaneous vessels extends
- Hands and feet come together and touch
Stage 22 of embryonic development
We are in the tenth week of pregnancy, 54 – 56 days after conception, the embryo is 23 – 28 mm long.
Characteristic signs of this stage:
- Development of the eyelids and external auditory canal
- Tragus and antitragus of the external auricle
- Nasal septum
- Secondary palate
Stage 23 of embryonic development
This represents the final stage of embryonic development in the tenth week of pregnancy. We are 56 – 60 days from conception and the length of the embryo is 27 – 31mm.
Characteristic signs of this stage:
- The angle between the head and the trunk is 30°
- The head represents 50% of the size of the embryo
- The head is rounded, the extremities elongated and stand out clearly from the body
- ment
- nasal well
- Development of the primordia of the external genitalia
Kathryn Barlow is an OB/GYN doctor, which is the medical specialty that deals with the care of women's reproductive health, including pregnancy and childbirth.
Obstetricians provide care to women during pregnancy, labor, and delivery, while gynecologists focus on the health of the female reproductive system, including the ovaries, uterus, vagina, and breasts. OB/GYN doctors are trained to provide medical and surgical care for a wide range of conditions related to women's reproductive health.