|
Pregnancy is the process by which a mammalian female carries a live offspring from conception until it develops to the point where the offspring is capable of living outside the womb. It starts with conception, the process of fertilization to form a zygote, and ends in childbirth, miscarriage, or abortion.
In humans, pregnancy takes approximately 40 weeks between the time of the last menstrual cycle and delivery (38 weeks from fertilization). It is divided into three trimesters. The first trimester carries the highest risk of miscarriage, the unintentional abortion of a fetus. It is often a result of defects in the fetus, its parent, or damage caused after conception.
Euphemisms for pregnancy include 'expecting', 'bun in the oven', 'in the family way', 'knocked up' and 'eating for two.' (The term 'knocked up' is generally regarded as profane.)
Fertilization
The first step of pregnancy usually begins with sexual intercourse where male gametes or sperm are deposited into the vagina. The semen produced by the male contains not only sperm cells but also sugars, proteins and other substances to help keep the sperm viable. Human sperm generally survive for about 48 hours in the female body. Sperm have a long flagellum, which they use to swim; they are the only human cells with this property. These cells are haploid, having divided by meiosis from germ cells in the testes and possessing only one half of the chromosomes of ordinary body cells. Typically, between 100 million and 300 million sperm are released in one ejaculation.
Ova, or oocytes, are the haploid female egg cells, and their role is to fuse with one sperm cell to form a fertilized zygote which will then grow in the uterus to form a developing fetus. These cells are produced by meiosis in the ovaries and stay in a state of suspended animation until activated by hormonal changes in the woman's menstrual cycle. Typically, only one ovum is released during each menstrual cycle.
At ovulation, the fimbriae at the end of the fallopian tube move over the ovary to catch the released ovum. If fertilization takes place, the sperm usually meet the ovum in the fallopian tube, requiring the sperm cells to swim from the upper vagina through the cervix and across the length of the uterus before reaching the fallopian tube—a considerable distance compared to the size of the sperm cell.
Once there, the successful sperm swim towards the ovum and each attempts to fertilize it. Each sperm cell contains a small pouch of enzymes that it uses to break through the outer layer of the ovum in order to fuse with it. This can take up to twenty minutes. Once the ovum fuses with a single sperm cell, its cell membrane changes, preventing fusion with other sperm. The fusion of the ovum's and sperm's nuclei to form a diploid cell completes the first stage of pregnancy.
Alternative methods of fertilization, including artificial insemination and in vitro fertilization, are sometimes used in cases of infertility and by single women and lesbian couples.
Development
Pre-implantation At this point, there exists a single totipotent cell. Mitotic cell division is the next process to occur, where each cell divides to produce another diploid cell. The zygote divides to produce two smaller cells, called blastomeres, roughly every 20 hours. These cells get progressively smaller until about 4 divisions have taken place resulting in 16 individual cells. This cluster of 16 cells, called a morula, leaves the fallopian tube and makes its way to the uterus.
Post-implantation A blastocele is a small cavity on the center of the cells, and the developing cells will grow around this. There will be a flat layer of cells on the exterior of this cavity, and the zona pellucida will remain the same size as before. Cells are growing increasingly smaller to fit in. This new structure with a cavity in the center and the developing cells around it is known as a blastocyst.
The presence of the blastocyst means that two types of cells are forming, inner cell mass growing on the interior of the blastocele and cells growing on the exterior of it. In 24 to 48 hours, the blastocyst's barrier, the zona pellucida breaches. The cells on the exterior of the blastocyst begin excreting an enzyme which erodes epithelial uterine lining and creates a site for implantation.
The blastocyst also secretes a hormone called human chorionic gonadotropin which in turn, stimulates the corpus luteum in the mother's ovary to continue producing progesterone, which acts to maintain the lining of the uterus so that the embryo will continue to be nourished. The glands in the lining of the uterus will swell in response to the blastocyst, and capillaries will be stimulated to grow in that region. This allows the blastocyst to receive vital nutrients from the mother.
Pregnancy tests detect the presence of human chorionic gonadotropin.
Placental circulation system The cells surrounding the blastocyst now destroy cells in the uterine lining, forming small pools of blood which in turn stimulates the production of capillaries. This is the first stage in the growth of the placenta. The inner cell mass of the blastocyst divides rapidly, forming two layers. The top layer will become the embryo and cells from there will be used in the amniotic cavity. At the same time, the bottom layer will form a small sac. (If the cells begin developing in an abnormal position, an ectopic pregnancy may also occur at this point.)
Several days later, chorionic villi in the forming placenta anchor the implantation site to the uterus. A system of blood and blood vessels now develops at the point of the newly forming placenta, growing near the implantation site. The small sac inside the blastocyst begins producing red blood cells. For the next 24 hours, connective tissue will develop between the developing placenta and the growing fetus. This will later develop into the umbilical cord.
Cellular differentiation Following this, a narrow line of cells appears on the surface on the embryo. Its growth shows the fetus will undergo gastrulation, in which the three layers of the fetus, the ectoderm, mesoderm and endoderm, will develop. The narrow line of cells begin to form the endoderm and mesoderm. The ectoderm begins to grow rapidly as a result of chemicals being produced by the mesoderm. These three layers will give rise to all the various types of tissue in the body.
The endoderm will later form the lining of the tongue, digestive tract, lungs, bladder and several glands. The mesoderm will form muscle, bone and lymph tissue, as well as the interior of the lungs, heart, reproductive and excretory systems. It will also give rise to the spleen, and will be used in the production of blood cells. The ectoderm will form the skin, nails, hair, cornea, lining of the internal and external ear, nose, sinuses, mouth, anus, teeth, pituitary gland, mammary glands, eyes and all parts of the nervous system.
Approximately 18 days after fertilization, the embryo has divided to form much of the tissue it will need. It is shaped like a pear, where the head region is larger than the tail. The embryo's nervous system is one of the first organs to grow. It begins growing in a concave area known as the neural groove.
The blood system continues to grow networks which allow the blood to flow around the embryo. Blood cells are already being produced and are flowing through these developing networks. Secondary blood vessels also begin to develop around the placenta, to supply it with more nutrients. Blood cells will begin to form on the sac in the center of the embryo, as well as cells which will begin to differentiate into blood vessels. Endocardial cells begin to form the musculature which will become the heart.
At about 24 days past fertilization, there is a primitive S-shaped tubule heart which begins beating. The flow of fluids throughout the embryo will begin at this stage.
Human pregnancy Anatomical model of a human pregnancy
Duration Traditionally (according to Naegele's Rule), a human pregnancy is considered to last approximately 40 weeks (280 days) from the last menstrual period (LMP), or 38 weeks (266 days) from the date of conception. However, a pregnancy is considered to have reached term between 38 and 42 weeks. Babies born before the 37 week mark are considered premature, while babies born after the 42 week mark are considered postmature.
However, the average length of pregnancy depends on ethnic background of the mother (Caucasian women are more likely to have a longer pregnancy than other women) and if it is a first pregnancy (which tend to last longer than subsequent pregnancies). For example, a Caucasian woman's first pregnancy lasts an average 274 days from conception (288 days from the last menstrual period). [1] (http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=Abstract&list_uids=2342739)
An accurate date of conception is important, because it is used in calculating the results of various prenatal tests (for example, in the triple screen test). A decision may be made to induce labour if a baby is perceived to be overdue. The process of accurately dating a pregnancy is complicated by the fact that not all women have 28 day menstrual cycles, or ovulate on the 14th day following their last menstrual period.
Effects on the mother
By trimester First trimester: Hormonal changes are the strongest, causing symptoms like morning sickness, fatigue, mood swings and food cravings. Pregnancy is usually suspected two to three weeks after conception.
Second trimester: Morning sickness usually disappears. Abdomen begins to bulge, starting the "obvious phase" of pregnancy. Hyperpigmentation, including linea nigra may appear.
Third trimester: Mother may experience backaches due to increased strain. Spine arched in order to counteract weight.
Overall During pregnancy, the mother undergoes many physiologic changes, be they cardiovascular, renal, hematologic, metabolic or respiratory, changes that become very important in the event of complications.
The mother is the sole provider of nourishment for her unborn baby, and so her plasma and blood volume increase by 40-50% to accommodate the changes. This results in overall vasodilation, increased heart rate (15 bpm), stroke volume, and cardiac output, ultimately increasing cardiac capacity by 70-80mL. Diastolic blood pressure consequently decreases between 12-26 weeks, and increases again to prepregnancy levels by 36 weeks. If the blood pressure remains abnormal beyond 36 weeks, the mother should be investigated for preeclampsia, a condition that precedes eclampsia.
Hematologically, the increase in plasma volume causes an increase in red blood cell mass. The mother also becomes hypercoagulable due to increased liver production of coagulation factors.
Pregnant women often have the sensation of shortness of breath. However, pregnant women oxygenate well, and maintain a total lung capacity comparable to non-pregnant women, with an increase in tidal volume, but a decrease in residual volume. Pregnant women breathe faster, and so have compensated respiratory alkylosis.
The increase in plasma volume results in increased levels of adrenocorticotropic hormone (ACTH), antidiuretic hormone (ADH), aldosterone, and cortisol in the kidneys, ultimately increasing the glomerular filtration rate (GFR) by 50%, a change that subsides around 20 weeks postpartum. Pregnant women may also show more proteinuria (300mg/24 hrs) than non-pregnant women (150mg/24 hrs). While this is normal, if the number rises beyond the 300mg/24 hours threshold, this would suggest renal impairment. Fully half of pregnant women will show signs of glucosuria, which is normal. If the glucosuria is persistent, gestational diabetes should be suspected.
During pregnancy, both protein metabolism and carbohydrate metabolism are affected. One kilogram of extra protein is deposited, with half going to the fetus and placenta, and another half going to uterine contractile proteins, breast glandular tissue, plasma protein, and hemoglobin.
Carbohydrate metabolism is also altered. Pregnant women have lower fasting blood glucose, and progressive insulin resistance, mostly due to human placental lactogen (HPL), which interferes with the insulin use of the body, and peaks at 24-28 weeks.
Imaging, Monitoring and care In present society medical science has developed a lot of precedures to safeguard a healthy pregnancy.
On the first visit to her gynecologist, the pregnant woman is asked to carry out the antenatal record, which constitutes a medical history and physical examination. Lab tests are also carried out, including the complete blood count (CBC), rubella vaccine, HIV test, pap smear, and a urine test.
On subsequent visits, the gestational age (GA) is rechecked with each visit. Symphysis-fundal height (SFH; in cm) should equal gestational age after 20 weeks of gestation, and the fetal growth should be plotted on a curve during the antenatal visits. The third Leopold maneuver is used to determine the position of the baby. Blood pressure should also be monitored, and may be up to 140/90 in normal pregnancies.
Fetal screening is also used to help assess the viability of the fetus, as well as congenital problems. Genetic counseling is often offered for families who may be at an increased risk to have a child with a genetic condition. Amniocentesis at around the 20th week is usually done for women 35 or older to check for Down's Syndrome and other chromosome abnormalities in the fetus. Even earlier than amniocentesis is performed, the mother may undergo the triple test, nuchal screening, nasal bone, alpha-fetoprotein screening and Chorionic villus sampling, also to check for disorders such as Down Syndrome.
Imaging is another important way to monitor a pregnancy. The mother and fetus are also usually imaged in the first trimester of pregnancy. This is done to predict problems with the mother; confirm that a pregnancy is present inside the uterus; guess the gestational age; determine the number of fetuses and placentae; evaluate for an ectopic pregnancy and first trimester bleeding; and assess for early signs of anomalies.
Computerized tomography (CT) and X-rays are not used in first trimester imaging due to the ionizing radiation, which has teratogenic effects on the fetus. Instead, ultrasound is the imaging method of choice in the first trimester and throughout the pregnancy, since it emits no radiation, is portable, and allows for realtime imaging. Ultrasound imaging may be done at any time throughout the pregnancy, but usually happens at the 12th week (dating scan) and the 20th week (detailed scan).
A normal gestation would reveal a gestational sac, yolk sac, and fetal pole. The gestational age can be assessed by evaluating the mean gestation sac diameter (MGD) before week 6, and the crown-rump length after week 6. Multiple gestation is evaluated by the number of placentae and amnionic sacs present.
Complications Several complications can arise throughout pregnancy. In the first trimester, the two major potential problems are miscarriage and ectopic pregnancy. Another complication in the first trimester is vaginal bleeding, which occurs in nearly 25% of clinical pregnancies in the first 20 weeks. It can be the result of a miscarriage, molar pregnancy or gestational choriocarcinoma. If an expectant mother experiences bleeding in pregnancy and is Rh factor negative, she should be given an Rh immunoglobulin such as Rhogam, regardless of whether or not the bleeding proceeds to miscarriage.
Later in pregnancy, complications include gestational trophoblastic disease (GTD) and gestational choriocarcinoma.
Other complications include:
Childbirth Main article: Childbirth
Childbirth is the process in which the baby is born. It may come about naturally or be induced for medical reasons.
Postnatal Main article: Postnatal
For topics following on from a successful pregnancy and birth, see:
See also
External links | 
Feeds |
Contact