Pregnancy Week by Week
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Your Baby’s Progress
Once it has implanted, the embryo secretes chemicals that have two functions. First, they signal to your body that the embryo has arrived, and this triggers a number of changes in your body: Your ovulation cycle stops, the mucus in your cervix thickens, your uterine wall softens, and your breasts begin to grow. Second, your immune system is suppressed so that the embryo is not treated as foreign and rejected, but is allowed to grow.
In addition, an outer layer of the blastocyst develops into a protective cocoon around the embryo. This cocoon will create the rudiments of the placenta and the support system in which the embryo will grow- the amniotic sac (the watery balloon in which it will float), the chorion (a safety cushion around the amniotic sac), and the yolk sac (which will manufacture blood cells until the liver takes over). The chorion then grows finer like projections, the chorionic villi, which the cocoon burrows firmly into your uterine lining.
The Cells specialize
Throughout these early weeks, the embryo’s cells become more specialized. There are now three layers of them, each destined to create different organs of the body. The innermost layer forms a primitive tube that will later develop into the lungs liver, thyroid gland, pancreas, urinary tract, and bladder. The middle layer will become the skeleton, muscles (including the heart muscle), testes or ovaries, kidneys, spleen, blood vessels, blood cells, and the deepest layer of skin, the dermis. The outer layer will provide the skin, sweat glands, nipples (and breasts, if it is a girl), hair, nails, tooth enamel, and the lenses of the eyes. These three cell layers differentiate to create an entire human body.
The Embryo’s Support System
The villi of the growing placenta intermingle with the maternal blood vessels of the uterine wall in such a way that they eventually become surrounded by lakes of blood. Maternal blood flows in and around these spaces, and because it is divided by only a cell or two from fetal blood, exchange of nutrients and waste between fetus and mother can occur in this blood space. The placenta is a hormone factory pumping out hormones, such as human chorionic gonadotropin (hCG), that are designed to support a healthy pregnancy.
Until the sixth week, the embryo’s blood cells are supplied by the yolk sac; after the end of the third week, blood circulation is pumped by the baby’s own heart.
Fourteen weeks after your LIMP, all of your baby’s major organs have formed and his intestines are sealed in the abdominal cavity. He now starts to grow and mature.
Your Baby’s Progress
By the eleventh week of pregnancy, your baby is recognizable as a human being, and he is now called a fetus (offspring) rather than an embryo. His head is very large compared to the rest of his body; by 14 weeks it will be about one-third of his whole length.
His eyes are completely formed, although the eyelids are still developing and remain closed. His face, too, is completely formed. His trunk has straightened out and the first bone tissue and ribs appear. The fingers and toes have nails, and some hair may have grown.
His external genital organs are now growing, and often the gender of the baby is discernible on a sonogram. Internally, his heart beats between 110 and 160 times per minute and his circulatory system continues to develop. The fetus swallows amniotic fluid and excretes it as urine. His sucking reflex is establishing itself– his lips purse, his head turns, and his forehead wrinkles.
The muscles he will use after birth for breathing and swallowing are also being exercised. In fact, by the end of this month your baby will have discovered movement. He now begins to move vigorously, though you probably won’t be able to feel his movements until the fourth month.
Blood Cell Production
While your baby will continue to rely on the placenta for nourishment, oxygen, and the clearance of waste until he is born, a system of blood cell formation that will eventually support independent life is essential. Toward the end of this month, the yolk asc becomes superfluous as its task of producing blood cells is taken over by your baby’s developing bone marrow, liver, and spleen.
His Support system
The placenta is developing very quickly, ensuring that there is a rich network of blood be vessels to provide your baby with vital nourishment. Now the layers thicken and grow until the chorion and membranes cover the entire inner surface area of the uterus. The umbilical cord is now completely mature and consists of three intertwined blood vessels encased in a fatty sheath.
The large vein carries nutrients and oxygen-rich blood to the fetus, while the two smaller arteries carry waste products and oxygen poor blood from the fetus to the placenta. The umbilical cord is coiled like a ring because the sheath is longer than the blood vessels. This allows him plenty of room to man oeuvre without the risk of damaging his lifeline.
The second trimester starts at the 14th week of pregnancy. Your baby is steadily growing, and if you have a scan at this time it is possible to discern the baby’s sex. If it is felt necessary, around this time you will be offered various tests to rule out abnormalities. The length of the femur will be measured, as well as the diameter of the head; this latter measurement will be used to confirm the EDD.
Your Baby’s Progress
She is looking more human, with legs longer than arms and the parts of her legs in proportion. The skeleton continues to produce bone and those parts with sufficient calcium can be seen on X-ray.
The fetus now contains the same number of nerve cells as an adult. The nerves from the brain begin to be coated in a layer of protective fat (myelin). This is an important step in their maturation because it facilitates the passage of messages to and from the brain. Connections between nerves and muscles are established so that your baby’s well-formed limbs can move at the joints when muscles are stimulated to contract and relax. Now that her arms are long enough, her hands can grasp each other if they touch accidentally, and she can form fists.
However, movements are not yet under the control of the brain. Nor do they register with you at first because the fetus is not big enough to activate nerve endings on the uterine wall. Second-time mother tend to feel fetal activity sooner. The fetus’s external genital organs acquire a more distinctive appearance. A girl’s vaginal plate, the precursor to her vagina, is clearly developing, and a boy’s testes are at the deep inguinal ring and well on their way to descending into the scrotum.
Her Support System
The placenta is producing the increasing amount of chorionic gonadotropin, estrogen, and progesterone that are needed throughout pregnancy. It also produces an assortment of other hormones that maintain the health of the uterus and play an essential part in the growth and development of the mother’s breasts in preparation for lactation.
The placenta forms a barier against infection, although not against viruses such as rubella (German measles) and AIDS or poisons such as alcohol and nicotine. By the end of the 16th week, the placenta has grown to about half an inch (1cm) in thickness and three and a half inches (7-8 cm) across.
Growth will continue until at term it reaches a weight of 11 pounds (500gm), a thickness of an inch and a half (3cm), and a diameter of 8 to 10 inches (20-25cm). It is firmly attached to the uterine wall (usually the upper part).
Your baby has sufficiently increased in size at this time to have developed a nervous system and muscles capable of allowing him to move. Because he is still so small, he can swim up and down and be in any position at any time.
Your Baby’s Progress
Starting now, form 19 weeks after your LMP, your baby’s rapid growth rate (but not weight gain) starts to slow down, and he matures in other ways. He begins to build up his defence systems. A sheath begins to form around the nerves in his spinal cord to protect them from possible damage. He also has his own primitive immune system, with which he can partially defend himself against some infections.
To produce body heat and maintain his temperature, your baby needs specialized fatty tissue. This is provided by a substance known as brown fat, which started to form during the fourth month. Now, deposits of brown fat begin to build up in areas of his body such as his neck, chest, and crotch. This will continue until term. One of the reasons that premature babies are so vulnerable is that they have insufficient amounts of brow fat and so are unable to keep themselves warm.
His skin will continue to grow, although it will be red and wrinkled because there is so little fat underneath it. From this month on, he becomes increasingly plumper. The baby’s sebaceous glands become increasingly plumper. The baby’s sebaceous glands become active and produce a waxy, greasy substance known as the vernix caseosa, which provides his skin with a protective coating during its long immersion in the amniotic fluid.
Your baby’s body is also covered with fine hair called lanugo. As yet, no one is quite sure of its purpose, but it may help to regulate his body temperature or it may be there to hold the protective vernix caseosa in place.
As his nerve fibers become connected and his muscle development and strength increase, his movements are more purposeful and coordinated. He embarks on his own athletic program- stretching, grasping, turning – which builds up his muscles, improves his motor ability, and strengthens his bones. These movements can make your abdomen sore.
A boy’s scrotum is solid at this stage. A girl’s vagina starts to become hollow, and her ovaries contain about 7 million ova, which will decline to approximately two million by the time she is born. By the time she reaches puberty, 200, 000 to 500, 000 ova will be left, and she will release only 400 to 500 of these during her adult life – approximately one per month.
Your baby is growing taller and stronger, while her movements are becoming more complex. She is also showing signs of sensitivity, awareness, and intelligence. A baby is legally viable if born after 24 weeks of pregnancy and may survive with specialized neonatal intensive care.
Your Baby’s Progress
She is still red and skinny, but she will soon start to put on weight. Any extensive wrinkling of the skin is caused by a lack of subcutaneous fat and a relative abundance of skin. Her body is growing faster than her head, and by the end of this month her proportions are approximately those of a newborn. Her arms and legs have their normal amount of muscle, her legs and body are in proportion, and the center of her bones is beginning to harden.
The lines start to appear on the palms of her hands. The brain cells she will use for conscious thought now start to mature, and she begins to be able to remember and learn (in one experiment, babies in the uterus were trained to kick in response to a specific vibration). The genitals are now completely differentiated; if the baby is a boy, testosterone-producing cells in the testes increase in number.
Your baby can hear sound frequencies that are beyond your range; she moves more in response to high frequencies than to low ones and moves her body in rhythm with your speech. Starting this month, she will respond to drum beats by jumping up and down. Some mothers report having to leave concerts because their unborn babies would not settle down.
If she hears a piece of music frequencies she may discover that when she is grown up it is familiar to her- even if she can’t remember ever hearing it. Some musicians have said that they “knew” unseen pieces of music and later discovered that their mother played them during pregnancy. She can also learn to recognize her father’s voice from this month onward.
A baby whose father talks to her while she is in the uterus can distinguish father’s voice in a roomful of people immediately after she is born and will respond to it emotionally for example, if she is upset, she may stop crying and calm down.
Inside her lungs, air sacs are forming in ever-increasing numbers. They will continue to increase until eight years after birth. Around them, the blood vessels that will help her absorb oxygen and expel carbon dioxide are multiplying.
In addition, her nostrils have now opened, and she is beginning to make breathing motions with her muscles, so her system will have plenty of breathing practice before she is born.