Embryo-fetal development from fertilization to 20 weeks. We begin with menstruation and explain the reason for the difference of two weeks that occurs from the last menstrual period or rule and the date of fertilization. We then follow the embryonic and fetal development describing some anatomical, anthropometric and behavioral characteristics that are interesting.

Note: Until week 13 (first trimester) the embryo-fetal length is based on the measurement from the head to the rump (crown-rump length). As of week 14, the length is equivalent to his Height, since the measurement goes from the head to the feet, but this data is not used during pregnancy. Measurements of length and weight can vary greatly until week 16; thus, they are measurements that work very well to adjust the age of pregnancy (Gestational Age) but not to evaluate the actual weight and height of your baby.

Embryo-fetal development of the first 20 weeks

Week 1: The first week of pregnancy does not really exist since it is the week in which the last menstruation occurred prior to fertilization; however, the two weeks (14 days) that follow the first day of the last menstrual period are taken into account for the calculation of the age of pregnancy since it is the only verifiable date (date) to calculate the gestational age. The truth is that all modern obstetrics (including obstetric ultrasound) consider this period of “non-pregnancy” as if it were. Thus, the Probable Date of Birth and the Gestational Age of the fetus are calculated starting from the “First day of the last menstrual period“. The simplest way to calculate the probable date of delivery is the Naegele Rule. The Conceptional Age is of embryological, non-clinical use and strictly follows the development of the embryo in days.

In cases of assisted reproduction “in vitro fertilization” we assume the day of implantation of the embryo (Conceptional Age) as the 14th day of the cycle and therefore for the calculation of the Gestational Age, delivery date and last rule we go back 2 weeks in the time and we assign the resulting date as the Last Rule or Period to make all the relevant calculations throughout the pregnancy.

Week 2: At the end of the second week (around the 14th day of the menstrual cycle) ovulation occurs and that allows, if there is a sexual relationship without protection, the sperm and the ovum join in the fallopian tube (fertilization) to form a unicellular entity called a zygote . If more than one egg is released and fertilized, you can have multiple zygotes ( multiple pregnancy ). The zygote has 46 chromosomes – 23 of you and 23 of your partner. These chromosomes contain the genetic material that will determine the sex of the baby and features such as the color of the eyes and hair, height, facial features and – at least to some extent – intelligence and personality. Immediately after fertilization, the zygote will begin its journey into the uterus through the fallopian tubes. At the same time, it will begin to divide to quickly form a growing group of cells

Week 3: Three days after fertilization (3 days after ovulation), the zygote that has been divided many times and looks like a blackberry ( Morula ) reaches the uterus on its journey through the fallopian tube. The zygote – which has continued to divide and already has about 500 cells – is now known as a blastocyst . When it reaches the inside of the uterus, approximately at day 6 after fertilization, the blastocyst will make a burrow in the uterine wall (endometrium) to feed (implantation). The external surface of the blastocyst will give rise to the placenta and the internal cell cluster will give birth to the baby. At the end of this week (day 21 of the cycle) and still missing 7 days for the next period to appear, the pregnancy test ( B hCG) already begins to become positive (a good laboratory) and the embryo is just a small disk microscopic two layers. Ecographically in a 3-week pregnancy there is no evidence of pregnancy except suspicion for a spongy and thickened endometrium, nothing is seen, but many interesting changes are taking place

Week 4: At the end of this week should occur menstrual bleeding but this does not happen because you are pregnant (oops!), This is equivalent to the second week of embryonic development (Conceptional Age, embryo use). In a matter of 7 days we went from a cluster of cells to a well-differentiated structure with external cells (placenta and membranes) and internal cells (embryonic cluster of three sheets that will give rise to various organs and tissues). The organization and cell division is so rapid that by the end of this fourth week the basic rudiment of the baby is already forming, especially as it concerns the brain, the spinal cord, the musculoskeletal system and the cardiovascular system (among others). The period of greatest teratogenic susceptibility ( malformation by external agents ) of the baby begins. The embryo measures less than 1 mm (from the head to the “rump” or tail) and can not be seen by ultrasound, in the best case only the surrounding sac is seen ( pregnancy of 4 weeks ), the only which is observed is a small bag of a few millimeters surrounded by a white ring (double contour ring).

Week 5: Towards the end of week 5 the baby’s heart begins to beat and the embryo is already evident, via transvaginal ultrasound, within the gestational sac ( 1-3 mm in length). With sophisticated equipment and vaginally we can even hear the embryonic beats (pulsed spectral Doppler) at the end of this week, around 100 beats per minute, the heart ultrasonographically seems to occupy at least half of the baby ( ultrasound 5 weeks ). From the embryological point of view, the upper (cephalic) end of the spine (now known as the neural tube) closes at the end of this week and the embryo begins to acquire a crescent shape, in fact, it looks like an animal with tail. The central bulge is effectively the heart and occupies almost 25% of the embryo.

Week 6: The cephalic end (head) of the embryonic neural tube (Neuroporo Nnterior or Cefálico) closes determining or not the presence of the group of anomalies known as Neural Tube Defects or DTN, Encephalocele type. The growth is fast this week and the embryo measures about 4 mm . The basic facial features will begin to appear, including the rudiments of the eye, an opening for the mouth and the corridors that will make up the inner ear. The digestive and respiratory systems begin to form as well. The small blocks of tissue that will form the baby’s connective tissue, ribs and muscles develop in the baby’s midline. Four small buds will soon grow to become arms and legs. Follow the process of bending ventral acquiring a form of “C” very closed. Ultrasound is observed as a grain of a few millimeters ( 6-week ultrasound ) with cardiac activity but no body movements. At the end of this week the Neuroporum Posterior or Caudal (“tail”) is closed, otherwise the Spina Bifida, the most frequent variant of DTN, is generated. NTDs can be detected after week 10 gestational

Week 7 : The umbilical cord appears because the baby begins to separate from the walls of the gestational sac, sometimes it can be followed to the point where the primitive placenta is forming. The embryo at this time measures on average 11 mm – a little larger than the eraser of a pencil. The buds of the arms that appeared last week now look like little rackets, the lower buds (legs) are less obvious. The face of the baby takes more definition this week and can be seen a perforation for the mouth and tiny notches of nose and sketches of the ears. The first manifestations of brain development are evident through the formation of primitive brain vesicles, which can be evident by transvaginal scan (7-week ultrasound) making clear the differentiation of the baby’s extremities: the head (head pole) and the rump (caudal pole).

Week 8: The brain emits its first detectable brain waves, most notably at this moment are the movements of the embryo: it moves like a worm with small movements of contortion. The buds of the four limbs with better definition of the digital rays (the beginning of the development of the fingers and toes) and the formation of the cerebral vesicles are evident. The wrists, elbows and ankles are clearly visible and the baby’s eyelids begin to form. The ears, the upper lip and the tip of the nose become recognizable. The baby’s heart beats faster, 150 beats a minute – about twice the usual adult frequency, the heart acquires its 4 cavities and the development of the sense of smell begins. Ecographically we see an embryo of 17 mm with clear differentiation of the podal poles (rabito) and cephalic (head), sketch of the four limbs, the spine and embryonic movements ( 8-week ultrasound )

Week 9: The embryo begins to lengthen and the bulge of the cephalic pole (head) and the elongation of the 4 limbs begins to give a human aspect to the embryo. The embryonic tail shrinks, which makes it look less like a tadpole and more like a person. The baby’s head – which is almost half the size of his body – now crawls down into the chest. The nipples and hair follicles begin to form. The baby’s pancreas, bile ducts, gallbladder and anus are in place, the intestines begin to migrate into the umbilical cord through the umbilicus, giving rise to the so-called “physiological hernia” that will disappear in week 11. The internal reproductive organs, testes or ovaries, begin to develop. The ultrasound image begins to be more defined as a baby ( ultrasound of 9 weeks ) and it is becoming easier to detail embryonic structures that previously only described embryology by studying lifeless embryos. It measures on average 24 mm (it weighs about 2 gr ). The body movements and the 4 members are easily evident

Week 10 : Graduation week! with dance included: the embryo is called fetus in the following, mobility is much greater and includes body movements and the 4 limbs; It moves throughout the gestational sac filled with amniotic fluid (about 50 mL in total). Neurons continue to multiply very quickly (almost 250,000 new neurons every minute), the essential organs of the baby have already been formed and the fetus becomes more resistant to external injuries (end of organogenesis). The arms and legs are clearly formed and articulated and the fingers are separated from each other; begins the calcification of the bones of the body including the spine. The eyelids of the baby are evident and occlude the eyes, which are no longer seen as black spots on the sides of the head, but located in front in their final position. The outer ears begin to assume their final shape, and the teeth begin to sprout within the gums. The shape of the skull, ocular orbits, nasal bones and the development of the lower jaw give the baby its typically human facial profile. If your baby is a male (46, XY) the testicles will begin to produce Testosterone that will cause the penis and scrotal sac to develop; if it is female (46, XX) and there is no Testosterone the genitals will follow a path to the genitals of the woman. Ultrasound ( 10-week ultrasound ) the image is very similar to the embryological image, is the image of a “little person” of about 34 mm ( 4 grams ) that moves, touches, stretches, etc.

Week 11: For a week the baby has been acquiring a shape that is more and more similar to the baby that will be born (patients say that it looks like a Martian in this stage: the head is big compared to the body), it will multiply its weight about 30 times and his height about 15 times. The organs and structures are becoming larger and easier to visualize, the kidneys begin to produce urine and the bladder is full, the stomach is easily visible. The study of the central nervous system is very interesting and important, you can see the ventricles, the rudimentary cerebellum and the cerebral sickle with acceptable detail, the cerebral cortex is very thin and transparent to ultrasound. With high-resolution equipment, the 4 cardiac chambers can be seen, thus initiating the early structural study of the heart ( 11-week ultrasound ). It measures 43 mm (remember, it is measured from the head to the tail, not including the legs) and weighs 7 gr. Here we start the advanced ultrasound evaluation to rule out chromosomal problems in the baby by using the first trimester genetic echo scheme. During this study, early fetal sex can be established through the angle of the genital tubercle (> 90% success), but I suggest waiting about 4 more weeks before buying the baby’s things …

Week 12: The baby continues to grow and becomes stylized, the nails appear on the fingers and toes. It is becoming easier to see it using the transvaginal ultrasound route and in thin patients, the abdominal route. We can define the face better and we can even detail the crystalline of the eyes, the respiratory movements are already apparent and we observe a great variety of body movements and limbs. Despite all those movements the mother does not feel her baby yet, and she will start doing it sometime between weeks 16 to 20. It measures about 57 mm and weighs around 14 gr

Week 13 : By this time the baby can put a finger in his mouth, the eyelids remain fused to protect eye development. The ossification is greater and you can clearly see the long bones and ribs. The fingers and toes are seen in great detail. The transvaginal route begins to be abandoned because the baby is very large for this type of study, we will continue through the abdominal route in successive sonographic examinations. During the next 5 to 7 weeks we will enter a phase where the luxury of details decreases (the baby is very large for the vaginal route and is small for the abdominal route), especially if the patient is obese since body fat does not drive very well the ultrasound. It measures about 74 mm and weighs about 23 grams .

End of the First Quarter

From this moment we will talk about the size of the baby, even when we do not measure it, including the legs (as if it were standing) using a theoretical formula that approximates reality: we add the gestational age in weeks (or the length skull-buttocks) with the length of the femur multiplied by 2. Until now, the length of the skull is used as a primordial element in the assignment of the gestational age of the embryo-fetus

Start the Second Quarter now.

At this time we stop using the transvaginal route and all future studies will be done abdominally . From this moment on, the best measurements for the calculation of gestational age are the femur, the abdomen and the fetal head.

Week 14: The effect of hormones is evident this week. In men, the prostate gland develops. In girls, the ovaries move from the abdomen to the pelvis. The Meconium – which will become the first evacuation after the birth of the baby – begins to form inside your intestine, but will not evacuate (under normal conditions) until after birth and during its first 24 hours of life. Before the end of this week, the roof of the baby’s mouth (palate) will be completely formed. Ossification of the bones becomes noticeable and it is from this moment that skeletal problems can arise. Sex can be determined safely starting this week. It weighs about 43 gr and measures approximately 14 cm

Week 15 : The skin begins to form. In the beginning the baby’s skin is almost non-existent and transparent but progressively begins to thicken covering the arteries and veins and giving it the usual appearance, the hair of the eyebrows and the hair begins to form and take the pigmentation that the parents determine. The bone and bone marrow that make up the baby’s skeletal system continue their development and the ossification that became noticeable at week 14 is much more evident now, since some skeletal disorders can begin to be noticed if they exist. The baby’s eyes and ears almost have their final appearance and location. If the patient is under 35 years of age and has no history of chromosomal disease, we requested the study of Maternal serum risk during this week (and until week 21, 15-21 – practically in disuse) as a method of screening for chromosomal disorders and spine bifida. It weighs 70 gr and can measure about 15 cm. We assign the definitive sex in 100% of babies of 15-16 weeks, especially in thin patients.

Week 16: The anatomical and functional development already allows the baby to begin to express facial expressions: frown, wrinkle face, smile, etc. The baby can already make a fist and its activity is intense, the muscle mass is such that from this week the expectant mother can begin to feel her baby (she can already move the uterine wall from inside). The baby’s eyes begin to be sensitive to light. Your baby may have frequent episodes of hiccups. In the females, a million ovules have already formed in the ovaries. If there is an indication, we begin to practice amniocentesis from this moment and during the next 3 weeks (16-19 weeks). For this moment it weighs approximately 100 gr and measures about 16 cm

Week 17: The fat begins to accumulate in the subcutaneous tissue so that the skin is no longer transparent and the bony relieves and other facial features begin to acquire a less skeletal touch (it is no longer so similar to ET, that of the movie of the 70’s). The fat will provide energy and help keep the baby warm after birth. It weighs 140 gr and measures about 18 cm

Week 18: The baby begins to hear because the nerve endings of the auditory system make connection with the inner ear: now the baby can hear your mother’s heartbeat, the rumbling stomach or the blood moving through the cord umbilical. It is possible that loud noises scare the baby. Your baby can swallow from this week or at least it was before but could not be observed, now it is easy to see the swallowing movements by ultrasound. From this moment you can perform Anatomical studies of the Second Trimester to achieve an excellent evaluation of organs that previously did not look good, for example, the fetal heart. It weighs 190 gr and measures 20 cm

Week 19: A thin layer of hair (lanugo) and whitish fat (vernix caseoso) begins to cover and protect the skin of the baby, this has nothing to do with the diet of the mother (the vulgar says that it is due to the intake of cheeses) and its origin is in the fetal glandular cutaneous secretion. The baby’s kidneys produce enough urine to maintain adequate levels of amniotic fluid (the amniotic fluid is primarily baby urine) whose volume is regulated by fetal swallowing movements. The sense organs continue to develop and now the baby is more sensitive to light and his hearing is more acute, it is possible that the baby begins to recognize sounds from this moment and can keep them in his memory. The intestine continues to form (it is being channeled) and it is already possible to find meconium inside it. Weight: 240 gr and size 22 cm . The females have already accumulated 6 million ovules in their ovaries

Week 20: The midpoint in the way of your pregnancy. For this moment it is expected that all pregnant women can feel and recognize the movements of their baby that is already about 25 cm and weighs about 300 gr. Under the protection of the vernix, the baby skin continues to thicken and develop layers in its epidermis; however, it is not considered mature yet, this will occur around week 35. Your baby now has thin eyebrows, hair, and very well developed limbs. The shape and general proportions of the baby are completely human, hey, see light, move, swallow, urinate, perhaps begins to have memory, etc. But all his movements are reflexes without any objective and he still does not feel pain. The movements are fundamental so that there are no joint or bodily deformities. Any loss during these first 20 weeks will be called Abortion since the birth at this fetal age is uniformly lethal due to the structural absence of the respiratory portion of the lungs (alveoli).

Next 20 weeks