From the 3rd to the 9th month of pregnancy, the development of the fetus is essentially centered on the growth, in size and weight, of the future baby. Back on these few weeks rich in small (r) evolutions.

It can sometimes appear subtle to future parents, but the terminology used to describe the unborn child varies during pregnancy depending on its stages of development. Thus, from fertilization to the 8th week of pregnancy (to the 56th day of gestation more specifically), the future baby is described by the term embryo. It then evolves during embryogenesis in 2 stages:

• until the 4th week of pregnancy (6 weeks), the initial formation of the embryo begins with the zygote, the first cell resulting from the fusion of the sperm and the ovum. This gradually divides by the phenomenon of mitosis in order to allow the differentiation cells from which the embryonic button (or embryoblast) will be born. Once the embryoblast has appeared, the cluster of cells which until then constituted the embryo evolves into an embryonic disc (during the 2nd and 3rd weeks of pregnancy) which “rolls up” into a cylindrical structure from the 4th week. pregnancy in order to circumscribe the embryo and definitively differentiate it from its appendages (future placenta, etc.). It’s here delimitation which marks the end of embryogenesis.
• until the 8th week of pregnancy (or 10 weeks), organogenesis marks the time of the roughing of all the organs of the embryo. Vital, sensory organs, tissues, bones: all gradually take their place and sometimes even their final form before being able to continue their growth during the second major phase of fetal development. This is the fetal phase.

The second trimester of pregnancy (from 14 to 28 weeks) is essentially marked by a phenomenon of strong growth of the fetus. Thus, the size of the fetus will almost triple in the space of these few weeks from 12 to 30 cm, its weight will be multiplied by 13 (from 45 to some 600 g)!

Other notable events of this period:

• The baby’s intrauterine movements intensify, to the point of being perceived by the future mother, generally between 16 and 18 weeks old, sometimes earlier for multiparas. Their interest: to allow the good conformation of its members. His actions then become more precise over the weeks and as the nervous system and the muscles of the fetus develop.
• his senses are refined. From the 4th month of pregnancy, he can hear his mother’s organic noises very well (heartbeats, etc.). Then around 25 WA, his auditory nerve and inner ear finished forming. He also develops his sense of touch (and thus becomes sensitive to caresses), his taste and even his sight from the start of the second trimester. However, her eyelids did not begin to separate until the end of the 6th month, around 28 weeks.
• His respiratory system is evolving:around 26 weeks her pulmonary alveoli are formed, the bronchial tree fills with amniotic fluid, and the bronchi continue to subdivide. However, her lungs will not mature until the 8th month of pregnancy.
• the kidneys that have set up in the 3-month-old fetus, become functional and therefore contribute to the composition and regulation of amniotic fluid and through this, lung maturation.
• The nervous system develops in several stages:

  – between 8 and 20 weeks of pregnancy (10 and 22 weeks), neurons multiply,

  – between 10 and 22 weeks of pregnancy (10 and 24 weeks), the cerebral cortex is organized in columns and layers,

  – from 23 weeks of pregnancy (25 weeks), neuron maturation takes place, not to end until several years after birth.

During the last trimester of pregnancy, the growth of the fetus slows down. Due to a lack of space in his mother’s womb, he thus “only” takes about fifteen centimeters in 3 months, going from 32 cm at 7 months to 45 cm at the beginning of the 9th month. On the other hand, her weight increases significantly, going from 1 kilo to more than 3 kilos at birth, at a weekly rate, during the last month of pregnancy, of 200 g.

Beyond this phenomenon, this last phase of pregnancy is also (in particular) marked by:

• The evolution of reproductive organs. Towards the end of the 7th month of pregnancy, the testes of the male fetus descend from the abdomen to the bursa. In girls, the ovarian reserve is constituted. After the gradual degradation of the millions of oogonia (primitive female sex cells) contained in the ovaries, 400 to 500 cells continue to mature. They then transform into primordial oocytes surrounded by primordial follicles, which later mature as part of the menstrual cycle.
• It is also around 30 SA that is defined presentation of the fetus. This positioning in the uterus contributes in part to the good progress of childbirth. In the vast majority of pregnancies (97%), the presentation is said to be cephalic (head down), even if certain presentations may favor (or even make essential) the use of a cesarean section (presentation of the face, forehead, transverse, etc.). Note: breech presentation does not make vaginal birth impossible, the mode of delivery then being assessed on a case-by-case basis depending on the state of health of the mother and child and the practices of the team. medical.
• Maturation of the respiratory system. At 34 WA, lung maturation is considered complete, although the lungs continue to develop until birth. Indeed, at this stage, the fetus produces surfactant, a substance necessary to keep the pulmonary alveoli of the newborn baby open after its first breath.
• The development of the nervous systemx continues with the myelination of nerves. The last stage of brain maturation, it begins around 34 weeks of pregnancy… and ends when the child comes of age. Its role: to facilitate the conduction of nerve impulses at the cerebral level.
• The maturation of the senses. The further the pregnancy progresses, the greater the sensory skills of the fetus. So around 28 weeks of pregnancy, he performs his first motor responses to auditory stimuli (he blinks when he hears noise, for example). Identically, the observation of premature babies made it possible to confirm that from 26 weeks of pregnancy, the baby can react to olfactory stimuli.

During pregnancy, 3 major ultrasound appointments are set for each trimester. Their goal: to monitor the growth and development of the fetus.

More particularly:

• between 11 WA and 13 WA + 6 days: the dating ultrasound aims to date the onset of pregnancy, identify a possible multiple gestation, assess the risks of chromosomal abnormalities (more particularly trisomy 21) and other pathologies.
• between 20 and 25 weeks: morphological ultrasound aims to continue this prenatal screening for diseases.
• between 30 and 35 WA: the 3rd ultrasound allows you to locate the placenta, identify possible intrauterine growth retardation (IUGR) and continue to look for possible fetal pathologies or abnormalities.

If prenatal screening always involves ultrasound control, it is also supplemented by other examinations (particularly serological) to identify the origin of a suspected pathology and confirm the diagnosis. The following are thus sought during prenatal follow-up:

Chromosomal abnormalities

Fetuses with chromosomal malformations often have morphological damage identifiable on ultrasound examinations. However, the diagnosis of chromosomal abnormality is based on the performance of a karyotype and a Chromosome Analysis on DNA Chip (ACPA). These anomalies can be linked:

• the number of chromosomes (aneuploidies) like trisomy 21, trisomy 13, trisomy 18, Klinefelter syndrome, triploidy, Turner syndrome, etc.
• to the structure of chromosomes such as 4p (Wolff-Hirschhorn syndrome) or 5p (cat cry syndrome) monosomies, Williams syndrome, Potoki-Luski syndrome, etc.

Genetic abnormalities

Some fetal damage may be due to the mutation of a single gene and around 6000 genes have been identified as implicated in human diseases to date. We then speak of monogenic diseases, their frequency fortunately remaining rare. Their mode of transmission depends on the associated gene (localization on one of the chromosomes, sexual or not) and on its mutation (one or two mutated alleles), which may partly explain the very variable nature of screening during pregnancy. These diseases include cystic fibrosis, sickle cell disease, fragile X syndrome or even Huntington’s disease.

Morphological anomalies …

… And more particularly digestive, cerebral, genitourological and cardiac disorders also have very variable origins and prevalence.

Anomalies of infectious origin

They are mainly due to:

• a bacteria : streptococcus B, colibacillus (E. Coli), listeria (lysteriosis), treponema pallidum (syphilis),
• A virus such as maternal transmission of rubella (at the origin of Gregg’s syndrome), herpes, cytomegalovirus or even chickenpox.
• a parasite such as toxoplasmosis which can be the cause of hydrocephalus in the unborn child.

The diagnosis of these anomalies combines ultrasound screening and serology.

From 36 weeks of pregnancy (38 WA), the fetus is no longer considered to be at risk of prematurity. The vast majority of her organs are fully developed and the remaining weeks of pregnancy are essentially an opportunity for the unborn baby to continue to develop adipose tissue … and therefore to grow.

However, even after the pregnancy has come to term, certain organs continue to mature during the postnatal period, namely:

• the brain with the continued myelination of nerves and the organization of neurons for several years,
• lungs, the pulmonary alveoli multiplying again intensively during the first months of life, then more gradually up to 2 years.