Ch 12The fetus

Dr. Areefa AlbahriMidwifery department

Introduction The midwife's role in embryological and

fetal development is focused on health education for maternal and fetal well-being. This involves providing parents with information about the effects of maternal lifestyle, such as diet, smoking, alcohol, drugs and exercise, on fetal growth and development. Additionally, an understanding of fetal development is of value when a baby is born before term.

Time scale of development Embryological development is complex

and occurs from weeks 2–8; and includes the development of the zygote in the first 2–3 weeks after fertilization. Fetal development occurs from week 8 until birth. The interval from the beginning of the last menstrual period (LMP) until fertilization is not part of pregnancy. However, this period is important for the calculation of the expected date of birth. Figure 12.1 illustrates the comparative lengths of these prenatal events.

Summary of embryological and fetal development

0–4 weeks• Primitive streak appears• Primitive central nervous system forms• Heart develops and begins to beat• Covered with a layer of skin• Limb buds form• Gender determined..

4–8 weeks • Very rapid cell division • More body systems laid down in

primitive form • Blood is pumped around the vessels • Lower respiratory system begins • Head and facial features develop • Early movements • Visible on ultrasound from 6 weeks.

8–12 weeks • Rapid weight gain • Eyelids fuse • Urine passed • Swallowing begins • External genitalia present but

gender not distinguishable • Fingernails develop • Lanugo appears • Some primitive reflexes

present.

12–16 weeks • Rapid skeletal development • Meconium present in gut • Nasal septum and palate fuse • Gender distinguishable.

16–20 weeks

• Constant weight gain • ‘Quickening’ • Fetal heart heard on

auscultation • Vernix caseosa appears • Skin cells begin to be renewed.

20–24 weeks • Most organs functioning well • Eyes complete • Periods of sleep and activity • Ear apparatus developing • Responds to sound • Skin red and wrinkled.

24–28 weeks survival may

be expected if born

Eyelids open Respiratory

movements.

28–32 weeks Begins to

store fat and iron

Testes descend into scrotum

Skin becomes paler and less wrinkled.

32–36 weeks Weight gain 25 g/day Increased fat makes the

body more rounded Lanugo disappears from

body Nails reach tips of fingers Ear cartilage soft Plantar creases visible.

36 weeks–Birth •Birth is

expected •Shape

rounded •Skull formed

but soft and pliable.

The fetal circulation The placenta is the source of oxygenation,

nutrition and elimination of waste for the fetus. The ductus venosus which connects the umbilical

vein to the inferior vena cava The foramen ovale which is an opening between

the right and left atria The ductus arteriosus which leads from the

pulmonary artery to the descending aorta The hypogastric arteries which branch off from the

internal iliac arteries and become the umbilical arteries when they enter the umbilical cord.

The fetal circulation takes the following course:

Oxygenated blood from the placenta travels to the fetus in the umbilical vein. The umbilical veins divide into the portal vein in the liver,& the ductus venosus joining the inferior vena cava. Most of the oxygenated blood that enters the right atrium passes across the foramen ovale to the left atrium and the left ventricle, and then the aorta. The head and upper extremities receive approximately 50% of this blood via the coronary and carotid arteries, and the subclavian arteries respectively. The rest of the blood travels down the descending aorta. A little blood travels to the lungs in the pulmonary artery, for their development.

Adaptation to extrauterine life At birth, there is a dramatic alteration to

the fetal circulation and an almost immediate change occurs. The cessation of umbilical blood flow causes a cessation of flow in the ductus venosus, a fall in pressure in the right atrium and closure of the foramen ovale. As the baby takes the first breath, the lungs inflate, and there is a rapid fall in pulmonary vascular resistance.

Adaptation to extrauterine life The ductus arteriosus constricts due

to bradykinin released from the lungs on initial inflation. The effect of bradykinin is dependant on the increase in arterial oxygen. In the term baby, the ductus arteriosus closes within the first few days of birth.

These structural changes become permanent and become as follows:

The umbilical vein becomes the ligamentum teres

The ductus venosus becomes the ligamentum venosum

The ductus arteriosus becomes the ligamentum arteriosum

The foramen ovale becomes the fossa ovalis

The fetal skull The fetal head is large in relation to

the fetal body compared with the adult . Additionally, it is large in comparison with the maternal pelvis and is the largest part of the fetal body to be born.

Divisions of the fetal skull The skull is divided into the vault, the base

and the face. The base comprises bones that are firmly united to protect the vital centres in the medulla. The face is composed of 14 small bones which are also firmly united and non-compressible. The vault is the large, dome-shaped part above an imaginary line drawn between the orbital ridges and the nape of the neck.

The bones of the vault The occipital bone lies at the back of the

head. Part of it contributes to the base of the skull as it contains the foramen magnum, which protects the spinal cord as it leaves the skull. The ossification centre is the occipital protuberance.

The two parietal bones lie on either side of the skull. The ossification centre of each is called the parietal eminence.

The two frontal bones form the forehead or sinciput. The ossification centre of each is the frontal eminence. The frontal bones fuse into a single bone by 8 years of age.

• The upper part of the temporal bone on both sides of the head forms part of the vault.

Sutures and fontanelles

Fetal skull landmarks

diameters of the fetal skull

The longitudinal diameters are: The sub-occipitobregmatic (SOB)

diameter (9.5 cm) measured from below the occipital protuberance to the centre of the anterior fontanelle or bregma

The sub-occipitofrontal (SOF) diameter (10 cm) measured from below the occipital protuberance to the centre of the frontal suture

The occipitofrontal (OF) diameter (11.5 cm) measured from the occipital protuberance to the glabella

•

The mentovertical (MV) diameter (13.5 cm) measured from the point of the chin to the highest point on the vertex, slightly nearer to the posterior than to the anterior fontanelle

The sub-mentovertical (SMV) diameter (11.5 cm) measured from the point where the chin joins the neck to the highest point on the vertex

The sub-mentobregmatic (SMB) diameter (9.5 cm) measured from the point where the chin joins the neck to the centre of the bregm

Moulding The term moulding is used to

describe the change in shape of the fetal head that takes place during its passage through the birth canal. Alteration in shape is possible because the bones of the vault allow a slight degree of bending and the skull bones are able to override at the sutures.

Moulding This overriding allows a considerable

reduction in the size of the presenting diameters,

Additionally, moulding is a protective mechanism and prevents the fetal brain from being compressed as long as it is not excessive, too rapid or in an unfavourable direction. The skull of the pre-term infant

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