The fusion of the ovum and spermatozoon is called conception, fertilization, or impregnation, and initiates the beginning of a new life. The ovary only produces one ovum per month, but as many as 200,000,000 spermatozoa each 0.05 mm. are deposited in the vagina when coitus takes place. At this time the cervix secretes a flow of alkaline mucous that attracts the spermatozoa and in which they are propelled by the rotary movement of their tails. A limited number of spermatozoa reach the Fallopian tube; a few may penetrate, but only one fuses with the nucleus of the ovum.
Without exception fertilization takes place in the Fallopian tube, usually in the ampulla. By means of the sharp point on its head the spermatozoon penetrates the ovum.
The most likely period for conception to take place is immediately following ovulation which occurs 14 days previous to the next menstrual period. The majority of women conceive between the 10th and 18th day of the menstrual cycle; but it is believed that conception can take place earlier or later. Neither ovum nor spermatozoon are thought to be capable of fertilization for longer than 48 hours.
DEVELOPMENT OF THE FERTILIZED OVUM
While being propelled along the tube, segmentation or cell division takes place and the fertilized cell or zygote divides into 2-4-8-16 and so on until it consists of a ball of cells like a mulberry, known as the morula. Three to four days are required for the journey along the tube to the uterus, and as the part of the Fallopian tube through which it must pass has a diameter of 1 mm., the ovum must be very small to pass through it. A cavity or blastocele forms in the morula which now becomes known as the blastocyst.
EMBEDDING OF THE OVUM
Six to seven days may elapse before the fertilized ovum, now at the blastocyst stage is ready for embedding. It comes to rest on the endometrium. When the ovum burrows into the implantation cavity, slight bleeding may occur which might be mistaken for a scanty menstrual period. The endometrial cells heal the opening, and the embedding of the ovum is complete.
DEVELOPMENT OF THE EMBRYO
Three primitive layers in the ovum can now be differentiated, and from each layer particular parts of the foetus develop.
The ectoderm forms the nervous system, skin and certain lining mucosa.
The mesoderm forms bone, muscle, the circulatory system, and certain internal organs.
Mesoderm forms the mucosa of the alimentary tract, the epithelium of the liver, pancreas, lungs and bladder.
In the inner cell mass two cavities appear:
The area between these two sacs is comprised of ectoderm, mesoderm, entoderm, and is known as the embryonic area, from which the embryo develops. The amniotic sac contains and protects the embryo. The yolk sac provides nourishment for the embryo. The embryo is attached to the placenta by a broad band of mesoderm. By a complicated process of ‘folding off’ the amniotic sac surrounds the embryo, blood cells develop, and by the 6th week a primitive form of circulation is established, which at the 12th week is functioning completely.
DEVELOPMENT OF THE PLACENTA
During the third week, the ovum is completely covered with chorionic villi, which grow profusely and are known as chorion frondosum, which ultimately form the placenta.
The foetus develops its own blood, just as it develops its heart, brain, eyes, etc. and it must not be thought that maternal blood circulates in the foetus. There are four layers of tissue between foetal and maternal blood. Foetal and maternal blood do not mix.
From the 12th to the 20th week the placenta weighs as much as and even more than the foetus, and this is because it must deal with the metabolic processes of nutrition.
THE PLACENTA AT TERM
The placenta (afterbirth) is a round, flat mass, about 22.9 cm. in diameter, 2.5 cm. thick at the centre and weighing approximately one-sixth of the weight of the baby at term.
Blood from the foetal heart circulates through the foetus, and because of the need for oxygenation and replenishment it leaves the foetus and is carried by the arteries of the umbilical cord to the placenta. The umbilical arteries spread over the foetal surface of the placenta and subdivide until they terminate in the chorionic villi, which absorb from the mother’s blood the products of digestion; e.g. amino-acids, glucose, minerals, vitamins and probably fatty acids.
FUNCTIONS OF THE PLACENTA
The placenta is the means through which the foetus obtains its needs, and it not only selects and transports from the mother’s blood the substances necessary for foetal life and growth; it also changes some of these so that the foetus can utilize them. The efficiency of many placental functions depends on adequate uterine blood flow. Placental functions can be classified as:
(a) HCG (human chorionic gonadotrophin). This hormone forms the basis of the immunological and other pregnancy tests.
(b) Progesterone is produced by the placenta from about the 12th week.
(c) Oestriol is produced by the feto-placental unit from the 6th – 12th week.
(d) HCS (human chorionic somatomammotrophin – placental lactogen). This hormone is concerned with foetal growth.
THE FOETAL SAC
The foetal sac consists of a double membrane, the outer, chorion, the inner amnion. The foetus and liquor are contained within this sac.
The chorion is a thick, opaque, friable membrane, adherent to the decidua vera on its outer aspect, until the third stage of labour when it becomes detached during the expulsion of the placenta.
The amnion is a smooth, tough, translucent membrane, lining the chorion, from which it can be detached up to the insertion of the umbilical cord.
THE AMNIOTIC FLUID
The liquor is the fluid in which the foetus floats and is present in the amniotic sac from the earliest weeks of pregnancy. The amount increases until at term the quantity is from 500-1500mls. It consists of 99% water, is alkaline in reaction, andvarious mineral salts are present, including urea, which is derived from urine passed by the foetus.
It is clear, pale, straw-coloured fluid. The fluid distends the amniotic sac and allows for the growth and free movement of the foetus. It acts as a shock absorber, protecting the foetus from jarring or injury.
The umbilical cord extends from the foetal umbilicus to the foetal surface of the placenta. It is composed of an embryonic form of connective tissue, intermingled with a gelatinous substance known as Wharton’s jelly, and is covered with amnion. The cord carries two arteries. The umbilical arteries are empty after birth and can be felt as fibrous cords.
The umbilical vein contains pure blood returning to the foetus after having been oxygenated, and replenished in the placenta.
Length of cord: The average length of the cord is 55.8 cm. and if less than 38 cm. it is said to be short.
The ovum – During the first three weeks the whole structure, including the sac is known as the ovum.
The Embryo – From the 3rd-8th week the term ‘embryo’ is used
The Foetus – From the 9th wek until birth the term ‘foetus’ is used
The Baby – After birth the foetus is known as a baby.
The above are medical terms. Always refer to the ‘baby’ as a ‘baby’!
THE GROWTH OF THE UTERUS
The uterus grows at such a regular rate that it is possible, within limits, to estimate the period of gestation by its size.
EIGHTH WEEK – The uterus cannot yet be palpated abdominally.
TWENTIETH WEEK – The fundus is two fingers below the level of the umbilicus or 15.2 cm. above the symphysis pubis.
TWENTIETH-FOURTH WEEK – The fundus is at the upper margin of the umbilicus.
THIRTIETH WEEK – The fundus is midway between the umbilicus and xiphisternum.
THIRTY-SXITH WEEK – The uterus rises to its highest level and is in contact with the ziphisternum. At the 38th week the fundus sinks down to about the level of a 34 weeks’ pregnancy, and this is known as ‘lightening’.
FORTIETH WEEK – The uterus is ready to go into labour. The lower uterine segment is relaxed and stretched, the cervix is shortened and soft.
I would just like to thank Susan Ross for her beautiful book, which became my bible during my 4th pregnancy. After 2 inductions, epidurals, forceps, episiotomies, drips, drugs and stitches spread over my first 3 births, I felt that this was my last chance to have the kind of birth I have always dreamed of. Well let me tell you I barely put the book down. It seemed to be with me nearly everywhere I went. I am thrilled to tell you I had a beautiful experience birthing my 4kg. posterior baby boy, because I knew that I could. I had faith in my body, and all the positive affirmations throughout the pregnancy worked. I birthed my son myself, with 2 Midwives watching, lights dim, and music playing. After 5 months, I still feel on that hormonal high. I have never contacted any author before, but this time I had to. Also, after all these years of being passionate about childbirth and pregnancy I plan to become a doula. So thank you again for touching my life in such a beautiful way. Ann