Development Of The Embryo

Development of the Embryo

Seven days after fertilization the ovum enters the cavity of the uterus where it embeds itself in the endometrium. At this stage the fertilized ovum begins to differentiate into its various parts. A cavity which has formed within the cellular mass gradually enlarges to occupy most of the central space of the developing pregnancy. At the same time a very specialized group of cells (the inner cell mass) commences to grow rapidly and protrude into the cystic space which will eventually become the amniotic cavity, and the inner cell mass will develop into the foetus itself. When the pregnancy embeds itself in the lining of the uterus there is no foetus present—just a mass of cells which will later develop into a baby.

Development of the EmbryoThe trophoblast, which surrounds the embryo and is responsible for embedding it in maternal tissue, develops rapidly to form sponge¬like projections which erode further into maternal tissue, to obtain the increasing requirements of the rapidly growing pregnancy. The embryo will develop into not only the baby itself but also the umbilical cord, the placenta, the amniotic fluid, and the amniotic and chorionic membranes.

Size of the Baby during Pregnancy

2nd Week (this is the end of the 2nd week, i.e. at the time of fertilization on the 13th or 14th day of a normal 28-day cycle). The pregnancy consists of only one fertilized cell and is invisible to the naked eye.

3rd Week. During the 3rd week the fertilized ovum travels along the Fallopian tube and arrives in the cavity of the uterus, where at the end of the 3rd week it embeds in the lining of the uterine cavity. The total pregnancy is still not visible to the naked eye.

4th Week. The pregnancy is embedded and grows rapidly during this week so that by the 28th day, or the end of the 4th week, it is just visible to the naked eye. The corpus luteum in the ovary continues to mature and to increase its production of progesterone, so that menstruation is suppressed. Under the influence of progesterone the cells lining the inner surface of the uterus change into the highly secretory and active form which they continue to adopt throughout pregnancy. This is known as the decidua of pregnancy, forming a very thick and vascular lining to the uterus.

5th Week. At the end of the 5th week the fertilized pregnancy is approximately 2 mm. in length and is visible to the naked eye. The foetus, within the newly formed amniotic sac, is beginning to take shape into its major component parts. The spine is beginning to form and a rudimentary nervous system is just recognizable.

6th Week. During the 6th week the formation of the head is rapidly followed by the chest and abdominal cavities. The rudimentary brain is completed and a spinal column as well as a spinal cord is properly formed. The tail of the foetus becomes less noticeable and the limb buds begin to appear at the comers of the body. The heart is forming within the chest cavity and by the end of the 6th week the first simple rudimentary heart and circulation are beginning to function. Blood vessels are forming in the umbilical cord. The earliest parts of the stomach and intestine are formed within the abdominal cavity.

The face has not yet taken shape, but small depressions are appearing where the eyes and ears will be situated. The mouth and jaw are also beginning to develop.

The length of the foetus is approximately 6 mm.

7th Week. By the end of the 7th week the limb buds have grown rapidly and are now clearly distinguishable as arms and legs, at the ends of which clefts are appearing which will later separate into fingers and toes. Blood cells have formed within the circulation and blood vessels now extend into the head and throughout the body. The heart, although still a very simple structure, has started to beat with sufficient force to circulate cells through the blood vessels. The chest is properly formed but the lungs remain as tiny, solid organs one on each side of the mid-line. The intestine is almost completely formed but has not yet assumed its correct position. The liver and kidneys have developed but are small and incapable of functioning. The brain and the spinal cord are growing rapidly and, with the exception of nerves to the limbs and skin, are almost complete. The development of the head is proceeding at great speed and it is gradually assuming its final shape. At this stage, however, it is still bent forward on the chest and rather strange looking lumps are present over the back and base of the head. The depressions which are to become the canals of the ears have deepened and the inner parts of the ears are forming. The eyes themselves are also developing, although the skin over them (which is subsequently to become the eyelids) remains completely intact. The nose is not yet formed but apertures for the nostrils are appearing. Development of the jaws and mouth is continuing.

The length of the foetus is approximately I-J cm.

8th Week. All the major internal organs are now formed, although in a somewhat rudimentary state, and they enlarge and continue to progress towards assuming their permanent shape and position. The heart is beating strongly now that the circulatory system has been established in the foetus. The lungs have grown considerably but remain solid. This is the main time for growth of the eyes and the inner part of the ear, including the middle ear which is responsible for balance and hearing. There is no external part of the ear at this stage. The head of the foetus is still very large in proportion to the body. The face is now beginning to assume some recognizable characteristics with depressions present where the eyes are destined to be and black pigmentation apparent beneath the skin overlying them. The nostrils are formed. The two sides of both the upper and lower jaw have now fused, so that the mouth is recognizable. The limbs continue to develop and the presence of shoulders and elbows, as well as hips and knees, is becoming apparent. The first, but very tiny, movements of the spine begin.

The length of the foetus is approximately 2.2 cm.

9th Week. At the end of the 9th week the foetus has a more mature appearance, although the head is still bent forward on the chest. Development of the eyes is now complete, but the eyelids are still intact over them. Development of the inner ear continues. A nose has now appeared and the development of the mouth proceeds. The limbs continue to grow very rapidly and rudimentary hands and feet are now obvious with some differentiation of the ends of the limb buds into fingers and toes. Movements are more definite but not apparent to the mother for a long time yet.

The length of the foetus is approximately 3 cm. and its weight approximately 2 gm.

10th Week. By the end of the 10th week the eyes have grown considerably in size and are easily recognizable beneath the unformed eyelids. The inner part of the ear is completely formed and the external part of the ear (the pinna) is commencing to grow. The face is more recognizable and although the head is still large as compared with the rest of the body, the bumps over the back of the head and the upper part of the neck are now disappearing. All the internal organs continue to develop. The ankles and wrists have formed and fingers and toes are easily distinguishable, although they are still joined together by webbing.

The umbilical cord is properly formed and blood is circulating along its arteries and vein. The placenta has not yet formed but the chorionic villi over the area of implantation, which will eventually become the placenta, are growing and maturing.

The length of the foetus is 4-5 cm. and its weight is approximately 5 gm.

11th Week. At the end of the 11th week the foetus is easily recognizable as a small human baby, although the head is relatively large for the body and the limbs are relatively short, small and thin because few muscles have yet developed. The head continues to assume its more rounded shape and the face is developing rapidly. The eyes are completely formed and are continuing to grow. The inner ear and middle ear are completely developed and the external ear continues to enlarge and assume its adult shape. The limbs are growing rapidly and their various components are’ now easily recognizable, but the fingers-and toes are still joined together. Movements of the limbs and spine increase.

The ovaries or testicles have formed within the body and the external genitalia are developing but the eventual sex cannot yet be distinguished.

The heart is completely formed and a satisfactory circulation is present for blood ts being pumped to all parts of the body as well as through the umbilical cord to that part of the chorion which will eventually become the placenta.

The approximate length of the baby is 5.5 cm. (as long as a little finger) and its weight is 10 gm.

It is interesting to note that at the end of the 11th week of pregnancy all the essential organs of the foetus are formed and the majority of them are beginning to function. Although eleven weeks may sound a long time, it is only the time span between, for instance, 1 June and 17 August. It is during this time that congenital abnormalities may occur if some factor has been introduced that has the ability to interfere with the formation of a particular organ at a critical stage of its growth, so that its natural and normal development is disturbed. Once an organ has been properly formed it cannot come to any great harm, whatever happens to the mother or the foetus. It is important to realize that major congenital catastrophes can only result from factors which adversely affect the growth and development of the baby before the end of the nth week. Any factor operating after this time will probably have no adverse effect at all, or, at very worst, will only have a minute effect upon the development of any particular organ.

12th Week. The head continues to assume a more rounded form but is still bent forward. The face is now properly formed and eyelids are present. The external ear continues to develop. The internal organs continue their growth and development. Muscular develop-ment in the body and in the limbs increases the amount of foetal movement which originally began about the 8th week. The growth of the external genital organs continues.

The approximate length of the baby is 6-5 cm. and its’weight about 18 gm.

13th Week. At the end of the 13th week the pregnancy has reached the end of the third calendar month. The uterus is now distended by the pregnancy and measures approximately 10-2 cm. in diameter. It can be palpated in the lower abdomen as a very soft, cystic swelling arising out of the pelvis. The amniotic sac which is distending the uterus contains approximately 100 ml. of amniotic fluid and the growing foetus is suspended within this fluid. The foetus has plenty of room to move within the amniotic cavity and it only comes in contact with the amnion (and therefore the wall of the uterus) if it happens to bounce against it. The head is now rounded and, as the neck is fully formed, it moves freely on the trunk. The face is also formed with the mouth, nose and eyes properly developed. The external ear is also properly developed. The internal organs are fully formed but the lungs, liver, kidney and intestine continue to grow and mature. The external genital organs continue to develop and the sex of the infant is now obvious. Foetal movement increases but is not yet sufficiently vigorous for the mother to be aware of it.

The length of the baby is approximately 7.5 cm. and its weight approximately 30 gm.

By the end of the 13th week the baby is properly formed. If delivered it could not survive because although all the organs are present they have not yet matured sufficiently to perform the duties for which they are designed. Syphilis, rubella and rare tropical diseases are the only known infections which can now cross to the foetus and cause it any harm. There are several specific drugs which, if taken in normal doses from this stage of pregnancy, may cause abnormal growth of a particular organ, but there is no drug which can cause gross congenital abnormality unless it is taken in such large doses as to jeopardize the mother’s life.

The remainder of the pregnancy is designed not only to allow the foetus to grow to a size at which it is capable of independent survival, but also to give all the vital organs in the body sufficient time to mature and develop their highly complex processes which are essential for independent survival. It can be appreciated, therefore, that while the weight of the baby may be important, it is upon its degree of maturity that the survival of the foetus really depends.

16th Week. By the end of the 16th week the limbs are properly formed and all the joints are moving. Vigorous movements continue but are rarely felt by the mother. The fingers and toes are normal and finger¬nails and toenails are present. The head is still relatively large for the size of the body but fairly rapid growth continues to enlarge the body. Primary sex characteristics continue to develop and the sex of the infant is now obvious to the untrained observer.

A fine downy hair, lanugo, forms over the whole foetus including the face. The eyebrows and eyelashes start to grow.

The approximate length of the baby is 16 cm. and its weight about 135 gm.

20th Week. The baby is now growing rapidly in length and in weight. Hair appears on the head. Muscle is rapidly increasing in the limbs and very active movements can easily be observed and felt by the mother. The foetus has a relatively large amount of liquor amnii (amniotic fluid) in which it can move and rotate with ease.

The length of the infant is approximately 25 – 5 cm. and its weight about 340 gm.

24th Week. The foetus continues to grow and its vital organs are now sufficiently mature for it to survive for a short time if delivered prematurely. It is unlikely to maintain independent life for any length of time, however, as the lungs are inadequately matured. The arms and legs have a normal amount of muscle, although the baby remains rather thin because deposits of subcutaneous fat have not yet been laid down.

The approximate length of a child born at this stage is 3 3 cm. and its weight 570 gm.

28th Week. At 28 weeks the foetus is viable, which means that, according to legal definition, it is capable of a separate and independent existence and if prematurely born it must be registered. The definition of viability and the date of viability during pregnancy were laid down approximately one hundred years ago and nothing has happened in the intervening years which has altered the baby’s chance of survival before the end of 28 weeks of pregnancy. Occasionally babies delivered before the 28th week of pregnancy do survive with very highly specialized expert medical and nursing care and attention but they are very,few and very lucky. The chances of survival at 28 weeks are approximately 5 per cent, providing of course that the child has no congenital abnormality and its very delicate organs have not suffered any damage or injury at birth.

Before the 28th week the termination of pregnancy is abortion. This may be legally performed if the provisions of the Abortion Act 1967 are complied with. After the 28th week of pregnancy a woman is said to go into premature labour and it is not a legal offence to induce premature labour in any circumstance even at 28 weeks and one day, even though it may be known that the baby’s chances of survival are virtually nil.

The foetus is now covered with vernix, which is a greasy, adherent, cheese-like material over the whole of the baby’s skin, protecting it from becoming waterlogged by its continual immersion in the liquor amnii. The baby’s body has grown more than the head has grown during the past few weeks so that the head is now only slightly out of proportion to the size of the body.

The length of the baby is 37 cm. and the weight is 900 gm.

32nd Week. The child is now perfectly formed and its head is in proportion with its body. A large amount of vernix is present over the whole body. If the baby is delivered the eyes will open spontaneously, especially if the baby is inverted (that is facing downwards, not head downwards) and the chances of survival are approximately 15 per cent.

The amount of subcutaneous fat that has been deposited is increasing, but the child is still relatively thin. Its length is approximately 40-5 cm. and its weight 1-6 kg. It will usually be lying with its head towards the mother’s pelvis.

36th Week. At this stage the baby is almost fully mature and stands more than a 90 per cent chance of survival if delivered. The main reason why babies fail to survive if they are delivered at this stage is because the lungs may be insufficiently developed. A great deal of subcutaneous fat has been laid down. A large quantity of vernix is still covering the baby and small amounts may be present in the amniotic fluid. The baby should have assumed its permanent position as a cephalic or head presentation, and in approximately 50 per cent of women having their first baby it is at this stage that the head will descend into the pelvis. If the head does not descend or engage at the 36th week there is no need for concern. Women having their second or subsequent baby will notice that the head does not engage until the 40th week, or very frequently until the onset of labour. In a male infant the testicles have descended into the scrotum.

The length of the baby is approximately 46 cm. and its weight 2-5 kg.

40th Week. The average pregnancy is supposed to last 280 days, or 40 weeks, from the first day of the last normal menstrual period (or 266 days from conception) but it must be remembered that this is an average, not a rule. Some women will have normal pregnancies of shorter duration, whilst others will have normal pregnancies of longer duration, and similarly, some babies will mature and be ready for delivery before the 40th week whilst others will be slow to mature and will not be ready until after the 40th week. The time of delivery is discussed on page nz, and has nothing to do with the baby’s subsequent physical or intellectual ability. The mature baby is properly developed and has a large quantity of subcutaneous fat which has been deposited over the previous ten weeks and which gives its limbs and body a rounded appearance. The lanugo has disappeared, or almost disappeared, from the body but may still be present over the shoulders, arms and legs and occasionally on the forehead. The hair on the head varies in length from 2 to 4 cm. The body is covered entirely with vernix except for the mouth and on the eyes. The vernix is particularly thick on the skin creases of the groin and the elbow, as well as round the neck, under the armpits and behind the knees. The sclera, or white part of the eye, is fairly white in colour and the iris is blue. Babies are always bom with blue eyes, although occasionally the colour may change within a few minutes of delivery. The nails are properly developed and protrude to the end of the finger but not beyond. They are very soft and will not damage the skin by scratching during the first 24 hours of life.

The baby increases in weight by approximately 7 per cent per week from the 36th to the 40th week, that is about 1 per cent, or 28 gm. a day. The approximate length of the baby at term is 50 cm. and its weight approximately 3-4 kg.

When is a Baby Fully Formed?

The baby is fully formed by the end of the 12th week of pregnancy and from then onwards it only grows and matures. The process of maturing, however, is certainly as important as the process of growth. At 12 weeks a baby has all the appropriate organs but they are not yet sufficiently well developed to function satisfactorily or independently.

At the 28th week of pregnancy a baby is stated to have the ability to undertake a separate existence and the term ‘viable’ is used. Each day, however, that the child can remain in utero beyond the 28th week increases the chance of survival, because it becomes more mature each day. Its heart is more capable of controlling the altered circulation after birth; the brain is developing so that it is more able to control the fundamental functions of the body; the lungs are developing and are more able to exchange oxygen; the liver with all its complex metabolic processes is also developing and maturing with each day that passes. It is not, therefore, the size of the baby that is the most important factor, but its maturity, and it is upon this that its eventual survival depends. In other words, a baby who is 38 weeks mature stands a much better chance of surviving than a baby who is 34 weeks mature. And similarly a baby who is 34 weeks mature has a much better chance of surviving than a baby who is 30 weeks mature.

Some babies, for instance those of diabetic mothers, may be unduly large but this does not increase their chance of survival. A baby weighing 2-5 kg. who is 38 weeks mature stands a better chance of survival than a baby who is 34 weeks mature and weighs 3-6 kg. In the latter instance the vital organs of the baby, while not being completely mature, still have to provide all the oxygen and vital functions for a 3-6 kg. baby, whereas in the former instance the better developed organs have only to supply the needs of a 2-5 kg. baby.