Prenatal development of the fetus,fetal membranes

Prenatal developnent of fetus

1. Three periods are distinguished in the prenatal development of the fetus. Ovular period or germinal period —which lasts for first 2 weeks following ovulation. In spite of the fact that the ovum is fertilized, it is still designated as ovum. Embryonic period —begins at 3rd week following ovulation and extends up to 10 weeks of gestation (8 weeks post conception). The crown-rump length (CRL) of the embryo is 4 mm. Fetal period begins after 8th week following conception and ends in delivery. The chronology in the fetal period is henceforth expressed in terms of menstrual age and not in embryonic age. Prenatal development of the fetus.

2. LENGTH OF THE FETUS: To determine the length of the fetus, the measurement is commonly taken from the vertex to the coccyx (crown-rump length) in earlier weeks. While, from the end of 20th week onwards, the measurement is taken from the vertex to the heel (crown-heel length - CH) .

3. The length is more reliable criterion than the weight to calculate the age of the fetus. In the first trimester CRL (mm) + 6.5 = gestational age in weeks . Gestational age is the duration of pregnancy calculated from the first day of last menstrual period (LMP). It is greater than the postconception (fertilization) age by 2 weeks At term , the average fetal weight in India varies from 2.5 kg to 3.5 kg

4. There are three stages of fetal nutrition following fertilization: (1) Absorption : In the early postfertilization period, nutrition is stored in deutoplasm within cytoplasm and the very little extra nutrition needed is supplied from the tubal and uterine secretion. (2) Histotrophic transfer : Following nidation and before the establishment of uteroplacental circulation, nutrition is derived from eroded decidua by diffusion and later on from the stagnant maternal blood in the trophoblastic lacunae . (3) Hematotrophic : With the establishment of the fetal circulation, nutrition is obtained by active and passive transfer from the 3rd week onwards.

5. THE FETAL CIRCULATION The umbilical vein carrying the oxygenated blood (80% saturated) from the placenta, enters the fetus at the umbilicus and runs along the free margin of the falciform ligament of the liver . In the liver, it gives off branches to the left lobe of the liver and receives the deoxygenated blood from the portal vein . The greater portion of the oxygenated blood, mixed with some portal venous blood, short circuits the liver through the ductus venosus to enter the inferior vena cava (IVC) and thence to right atrium of the heart.

6. The O2 content of this mixed blood is thus reduced. Although both the ductus venosus and hepatic portal/fetal trunk bloods enter the right atrium through the IVC, there is little mixing. The terminal part of the IVC receives blood from the right hepatic vein. In the right atrium, most of the well oxygenated (75%) ductus venosus blood is preferentially directed into the foramen ovale by the valve of the inferior vena cava and crista dividens and passes into the left atrium.

7. Here it is mixed with small amount of venous blood returning from the lungs through the pulmonary veins. This left atrial blood is passed on through the mitral opening into the left ventricle . Remaining lesser amount of blood (25%) , after reaching the right atrium via the superior and inferior vena cava (carrying the venous blood from the cephalic and caudal parts of the fetus respectively) passes through the tricuspid opening into the right ventricle.

8. During ventricular systole, the left ventricular blood is pumped into the ascending and arch of aorta and distributed by their branches to the heart, head, neck, brain and arms. The right ventricular blood with low oxygen content is discharged into the pulmonary trunk . Since the resistance in the pulmonary arteries during fetal life is very high, the main portion of the blood passes directly through the ductus arteriosus into the descending aorta bypassing the lungs where it mixes with the blood from the proximal aorta.

9. 70% of the cardiac output (60% from right and 10% from left ventricle) is carried by the ductus arteriosus to the descending aorta. About 40% of the combined output goes to the placenta through the umbilical arteries. The deoxygenated blood leaves the body by way of two umbilical arteries to reach the placenta where it is oxygenated and gets ready for recirculation. The mean cardiac output is comparatively high in fetus and is estimated to be 350 mL /kg/min.

10. CHANGES OF THE FETAL CIRCULATION AT BIRTH The hemodynamics of the fetal circulation undergoes profound changes soon after birth due to cessation of the placental blood flow and initiation of respiration . The following changes occur in the vascular system 1. Closure of the umbilical arteries : Functional closure is almost instantaneous preventing even slight amount of the fetal blood to drain out. Actual obliteration takes about 2–3 months. The distal parts form the lateral umbilical ligaments and the proximal parts remain open as superior vesical arteries .

11. 2. Closure of the umbilical vein : The obliteration occurs a little later than the arteries, allowing few extra volume of blood (80–100 mL ) to be received by the fetus from the placenta. The ductus venosus collapses and the venous pressure of the inferior vena cava falls and so also the right atrial pressure. After obliteration, the umbilical vein forms the ligamentum teres and the ductus venosus becomes ligamentum venosum . 3. Closure of the ductus arteriosus : Within few hours of respiration, the muscle wall of the ductus arteriosus contracts probably in response to rising oxygen tension of the blood flowing through the duct. The effects of variation of the O2 tension on ductus arteriosus are thought to be mediated through the action of prostaglandins.

12. Prostaglandin antagonists given to the mother may lead to the premature closure of the ductus arteriosus . Whereas functional closure of the ductus may occur soon after the establishment of pulmonary circulation, the anatomical obliteration takes about 1–3 months and becomes ligamentum arteriosum . 4. Closure of the foramen ovale : This is caused by an increased pressure of the left atrium combined with a decreased pressure on the right atrium. Functional closure occurs soon after birth but anatomical closure occurs in about 1 year time . During the first few days, the closure may be reversible. This is evidenced clinically by the cyanotic look of the baby during crying when there is shunting of the blood from right to left.Within 1 or 2 hours following birth, the cardiac output is estimated to be about 500 mL /min and the heart rate varies from 120–140 per minute.

13. FIRST TRIMESTER (FIRST 12 WEEKS) SUBJECTIVE SYMPTOMS The following are the presumptive symptoms of early months of pregnancy: Amenorrhea: However, cyclic bleeding may occur up to 12 weeks of pregnancy, until the decidual space is obliterated. Morning sickness (Nausea and vomiting) is inconsistently present in about 70% cases, more often in the first pregnancy than in the subsequent one. It usually appears soon following the missed period and rarely lasts beyond 16 weeks. Diagnosis of Pregnancy

14. Frequency of micturition - It is due to resting of the bulky uterus on the fundus of the bladder because of exaggerated anteverted position of the uterus. Breast discomfort in the form of feeling of fullness and ‘pricking sensation’ is evident as early as 6–8th week specially in primigravidae . Fatigue is a frequent symptom which may occur early in pregnancy.

15. OBJECTIVE SIGNS: Breast changes Per abdomen Uterus remains a pelvic organ until 12th week , it may be just felt per abdomen as a suprapubic bulge. Pelvic changes Jacquemier’s or Chadwick’s sign : It is the dusky hue of the vestibule and anterior vaginal wall visible at about 8th week of pregnancy. The discoloration is due to local vascular congestion.

16. Vaginal sign : Apart from the bluish discoloration of the anterior vaginal wall. The walls become softened and Copious non-irritating mucoid discharge appears at 6th week. There is increased pulsation , felt through the lateral fornices at 8th week called Osiander’s sign.

17. Cervical signs : Cervix becomes soft as early as 6th week ( Goodell’s sign), a little earlier in multiparae . The pregnant cervix feels like the lips of the mouth, while in the non-pregnant state,like that of tip of the nose. On speculum examination, the bluish discoloration of the cervix is visible. It is due to increased vascularity .

18. Uterine signs: Size, shape and consistency — The uterus is enlarged to the size of hen’s egg at 6th week, size of a cricket ball at 8th week and size of a fetal head by 12th week. The pyriformshape of the non-pregnant uterus becomes globular by 12 weeks. Th ere may be asymmetrical enlargement of the uterus if there is lateral implantation. Th is is called Piskacek’s sign where one half is more firm than the other half. As pregnancy advances, symmetry is restored. The pregnant uterus feels soft and elastic.

19. Mantgomery tubercule

20. IMMUNOLOGICAL TESTS FOR DIAGNOSIS OF PREGNANCY Pregnancy tests depend on detection of the antigen ( hCG ) present in the maternal urine or serum with antibody either polyclonal or monoclonal available commercially. ULTRASONOGRAPHY: Intradecidual gestational sac (GS) is identified as early as 29 to 35 days of gestation (3-5 weeks) .

21. SUMMARY OF DIAGNOSIS OF PREGNANCY Positive or absolute signs: Palpation of fetal parts and perception of active fetal movements by the examiner at about 20th week Auscultation of fetal heart sounds Ultrasound evidence of embryo as early as 6th week and later on the fetus Radiological demonstration of the fetal skeleton at 16th week and onwards.

22. Presumptive symptoms and signs: It includes the features mainly appreciated by the women. Amenorrhea Frequency of micturition Morning sickness Fatigue Breast changes Skin changes Quickening.

23. Probable signs: Abdominal enlargement Braxton-Hicks contractions External ballottement Outlining the fetus Changes in the size, shape and consistency of the uterus Jacquemier’s sign Softening of the cervix Osiander’s sign Internal ballottement Immunological test.

24. Fetal Skull and Maternal Pelvis Fetal skull is to some extent compressible and made mainly of thin pliable tabular (flat) bones forming the vault. This is anchored to the rigid and incompressible bones at the base of the skull. SUTURES The sagittal or longitudinal suture lies between two parietal bones. The coronal sutures run between parietal and frontal bones on either side. The frontal suture lies between two frontal bones. The lambdoid sutures separate the occipital bone and the two parietal bones.

25. Importance: It permits gliding movement of one bone over the other during molding of the head, a phenomenon of significance while the head passes through the pelvis during labor. Digital palpation of sagittal suture during internal examination in labor gives an idea of the manner of engagement of the head ( asynclitism or synclitism ), degree of internal rotation of the head and degree of molding of the head.

26. FONTANELS: Wide gap in the suture line is called fontanel. Of the many fontanels (6 in number), two are of obstetric significance: Anterior fontanel or bregma and Posterior fontanel or lambda . Anterior fontanel: It is formed by joining of the four sutures in the midplane . The sutures are anteriorly frontal, posteriorly sagittal and on either side, coronal. The shape is like a diamond. Its anteroposterior and transverse diameters measure approximately 3 cm each . The floor is formed by a membrane and it becomes ossified 18 months after birth. It becomes pathological, if it fails to ossify even after 24 months.

27. Posterior fontanel: It is formed by junction of three suture lines — sagittal suture anteriorly and lambdoid suture on either side. It is triangular in shape and measures about 1.2 Ч 1.2 cm (1/2" Ч 1/2"). Its floor is membranous but becomes bony at term. Thus, truly its nomenclature as fontanel is misnomer. It denotes the position of the head in relation to maternal pelvis.

28. MOLDING: It is the alteration of the shape of the fore-coming head while passing through the resistant birth passage during labor. Molding disappears within few hours after birth. Vertex presentation with well flexed head; (B) Vertex presentation with deflexed head (sugar loaf head); (C) Face presentation; (D) Brow presentation

29. CAPUT SUCCEDANEUM: It is the formation of swelling due to stagnation of fluid in the layers of the scalp beneath the girdle of contact. The girdle of contact is either bony or the dilating cervix or vulval ring. The swelling is diffuse, boggy and is not limited by the suture line . It disappears spontaneously within 24 hours after birth . It may be confused with cephalhematoma .

30. Lie Definition: Relationship between the longitudinal axis of fetus and mother longitudinal (resulting in either cephalic or breech presentation) oblique (unstable, will eventually become either transverse or longitudinal) transverse (resulting in shoulder presentation)

31. In obstetrics, the  presentation  of a fetus about to be born specifies which anatomical part of the fetus is leading, that is, is closest to the pelvic inlet of the birth canal. According to the leading part, this is identified as a  Cephalic Breech Shoulder  A  malpresentation  is any presentation other than a vertex presentation (with the top of the head first).

32. Vertex presentation with longitudinal lie: Left occipitoanterior (LOA)—the  occiput  is close to the vagina (hence known as vertex presentation), facing anteriorly (forward with mother standing) and toward the left. This is the most common position and lie. Right occipitoanterior (ROA)—the occiput faces anteriorly and toward the right. Less common than LOA, but not associated with labor complications. Left occipitoposterior (LOP)—the occiput faces posteriorly (behind) and toward the left. Right occipitoposterior (ROP)—the occiput faces posteriorly and toward the right.

33. PELVIS The pelvis is anatomically divided into a false pelvis and a true pelvis , the boundary line being the brim of the pelvis. The bony landmarks on the brim of the pelvis from anterior to posterior on each side are—upper border of symphysis pubis , pubic crest , pubic tubercle , pectineal line , iliopubic eminence , iliopectineal line , sacroiliac articulation , anterior border of the ala of sacrum and sacral promontory

34. 1-Symphysis pubis. 2-Pubic crest. 3-Pubic tubercle. 4-Pectineal line. 5-Iliopubic eminence. 6-Iliopectineal line. 7-Sacroiliac articulation. 8-Anterior border of the ala of sacrum and 9-Sacral promontory

35. Inclination: In the erect posture, the pelvis is tilted forward. As such, the plane of the inlet makes an angle of about 55°with the horizontal and is called angle of inclination . Another way of measuring the inclination radiographically is to take the angle between the plane of the inlet and the front of the body of the fifth lumbar vertebra. The angle is normally about 135°

36. Pelvimetry  is the measurement of the female pelvis.  It can theoretically identify  cephalo -pelvic disproportion , which is when the capacity of the pelvis is inadequate to allow the fetus to negotiate the birth canal. However, clinical evidence indicate that all pregnant women should be allowed a trial of labor regardless of pelvimetry results.

37. Obstetric conjugate It is the distance between the midpoint of the sacral promontory to prominent bony projection in the midline on the inner surface of the symphysis pubis . The point is somewhat below its upper border. It is the shortest anteroposterior diameter in the anteroposterior plane of the inlet. It measures 10 cm . It cannot be clinically estimated but is to be inferred from the diagonal conjugate—1.5–2 cm to be deducted or by lateral radiopelvimetry . Diagonal conjugate It is the distance between the lower border of symphysis pubis to the midpoint on the sacral promontory. It measures 12 cm.

38. PELVIC AXIS: Anatomical (curve of Carus ): Anatomical pelvic axis is formed by joining the axes of inlet, cavity and outlet . It is uniformly curved with the convexity fitting with the concavity of the sacrum. The fetus does not, however, transverse the uniform curved path. Obstetrical: It is through this axis that the fetus negotiates the pelvis. It is not uniformly curved. Its direction is first downward and backward up to the level of ischial spines and then directed abruptly forward.

39. Terminology a nullipara is one who has never completed a pregnancy to the stage of viability. She may or may not have aborted previously. a nulligravida is one who is not now and never has been pregnant. a primipara is one who has delivered one viable child. Parity is not increased even if the fetuses are many (twins, triplets). a primigravida is one who is pregnant for the rst time. a multigravida is one who has previously been pregnant. She may have aborted or have delivered a viable baby. multipara is one who has completed two or more pregnancies to the stage of viability or more. a parturient is a women in labor. a puerpera is a woman who has just given birth.