Pregnancy - getting pregnant, fertility, birth, conception
Pregnancy is period of time between fertilization of the ovum (conception) and birth, during which mammals carry their developing young in the uterus (see embryo). The duration of pregnancy in humans is about 280 days, equal to 9 calendar months. After the fertilized ovum is implanted in the uterus, rapid changes occur in the reproductive organs of the mother.
The uterus becomes larger and more flexible, enlargement of the breasts begins, and alteration of renal function, blood volume, and blood cell count occur. Movement of the fetus and fetal heartbeat can be detected early in pregnancy.
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 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.
At this point, there exists a single totipotent cell with the potential to create an entire human being. Mitotic cell division is the next process to occur, where each cell doubles 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.
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). 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.
One test that has been used to determine pregnancy uses blood or urine samples to detect a hormone known as BhCG, found exclusively in pregnant women. Later, prenatal diagnostic tests such as alpha fetoprotein, amniocentesis, and chorionic villus sampling may be performed as screening measures for congenital defects. Ultrasound, a sonar device using high-frequency wavelengths, is used to detect defects, measure fetal heartbeat, and monitor growth of a fetus.
Complications of pregnancy include eclampsia, premature birth, and erythroblastosis fetalis (Rh incompatibility). Ectopic pregnancy, in which the fetus begins to develop outside the uterus, often in a fallopian tube, is another complication. It is often the result of scarring from a sexually transmitted disease. Smoking has been linked to low-birth weight infants; alcohol consumption during pregnancy has been linked to a group of defects called fetal alcohol syndrome.
The technology relating to pregnancy has made great advances and has created a number of ethical issues. Many women in their 40s are now able to sustain successful pregnancies, due to technological devices that carefully monitor the progress of the fetus. In vitro fertilization and other infertility treatments have allowed even postmenopausal women to give birth. The use of fertility drugs has led to a marked increase in multiple births. Abortion, in which pregnancy is terminated prior to birth, has long been a subject of heated debate, and surrogate motherhood (see surrogate mother) has also raised ethical issues in recent years.
More information on pregnancy
Pregnancy - Pregnancy is period of time between fertilization of the ovum (conception) and birth, during which mammals carry their developing young in the uterus (see embryo).
Pregnancy signs and symptoms - During pregnancy a woman's body undergoes a number of changes to allow the fetus to develop inside the womb. The symptoms of pregnancy vary from woman to woman.
Pregnancy tests - A pregnancy test is a test of blood or urine used to determine whether a woman is pregnant. There are two types of pregnancy tests - blood and urine tests.
Home pregnancy test - A home pregnancy test measures the presence of the hormone human chorionic gonadotropin (hCG) in your urine. All home pregnancy test kits test your pregnancy on the basis of your urine sample.
Pregnancy stages - There are three stages of pregnancy called trimesters. Each trimester is three months. The word "trimester" comes from a Latin word meaning "three months long."
First trimester of pregnancy - First trimester pregnancy is the early stage of pregnancy from conception to 12 weeks gestation, or about 14 weeks from the first day of the last normal menstrual period (LNMP).
Second trimester of pregnancy - In the second trimester the embryo, now known as a fetus, is recognisable as human in form, but is not developed enough to be viable if born. The second trimester is often called the planning trimester.
Third trimester of pregnancy - The third trimester of pregnancy lasts from 28 weeks after your last menstrual period (LMP) until the birth, which usually occurs between the 38th and 42nd weeks of pregnancy.
Calculating pregnancy due date - The due date is usually computed from the first day of the last regular period. In the calendar, this can be figured by taking that date, subtracting three months, and adding seven days.
Prenatal diagnosis - Prenatal diagnosis is the process of detecting and diagnosing fetal abnormalities before birth. A targeted prenatal diagnosis is done when there is a concrete suspicion that there might be a particular disorder.
Healthy pregnancy diet - A balanced diet is key to having a healthy pregnancy. Pregnancy places substantial demands on the availability of iron in the body.
Nutrition during pregnancy - Nutrition is an essential component of prenatal care. During pregnancy, your body needs more nutrients in order to provide a baby with what it needs. Eat enough food to gain weight at the rate recommended by your health care provider.
Exercise during pregnancy - Exercise plays an important role in promoting health and well-being for pregnant women. Excessive levels of physical activity in pregnancy can reduce fetal growth and increase the risk of preterm delivery.
Spotting during pregnancy - Spotting is light bleeding similar to your period and it can happen at any time during pregnancy, but it is most common during the first trimester.
Bleeding during pregnancy - Bleeding from the vagina in early pregnancy is very common. First trimester bleeding is any vaginal bleeding during the first 3 months of pregnancy.
Smoking during pregnancy - Cigarette smoking during pregnancy can cause serious health problems to an unborn child. Many complications of pregnancy are more likely to occur in smokers.
Sex during pregnancy - Sex and sexual intercourse are not harmful during pregnancy. For most women and their partners, sex during pregnancy is fine as long as both partners consent and are comfortable.
Prenatal care - Prenatal care is the health care that a woman receives before her baby is born. Prenatal care is provided for women during the period between conception and birth of the baby.
Teenage pregnancy - Teenage pregnancy is a pregnancy that occurs in an adolescent. Babies born to teenagers are at risk for neglect and abuse.
Twins and multiple birth - Multiple pregnancies are on the rise in recent years with more and more twins and other types of multiples being born. A multiple birth is when more than one human baby results from a single pregnancy.
Childbirth - Childbirth (also called labour, birth, or parturition) is the culmination of pregnancy, the emergence of a child from its mother's uterus.
Obstetrics - Obstetrics is the surgical specialty dealing with the care of a woman and her offspring during pregnancy, childbirth and the puerperium (the period shortly after birth).
Pregnancy ultrasound - Pregnancy ultrasound is a method of imaging the fetus and the female pelvic organs during pregnancy.
Chinese lunar pregnancy calendar - The Chinese pregnancy calendar was allegedly discovered about 700 years ago. The accuracy of the chart has been proved by thousands of people and is believed to be 99 percent accurate.
Fertility charting - Fertility charting allows you to predict ovulation, pinpoint your most fertile time in your cycle, and increase your chances of becoming pregnant.
Ovulation: calendar, prediction, test - Ovulation is the process of discharging a mature ovum (egg) from an ovary after a Graafian follicle - representing the final stage of follicular development before ovulation - has been formed.
Getting pregnant - The best or most fertile time to get pregnant is the period of ovulation in your menstrual cycle. Most women ovulate (release an egg from the ovary) about two weeks before their period.
Gender selection - There are three main techniques of sex selection: pre-natal testing and termination of pregnancy, pre-implantation genetic testing of embryos, and sperm sorting.
Prenatal tests - Prenatal tests are one of the many ways your practitioner can check on the well-being of your growing baby and find out whether you're at risk for complications.
Genetic screening - Genetic screening is a process used to find out what diseases or birth defects a child might inherit from his or her parents.
Genetic counseling - Genetic counseling is the process of determining the risk you have of passing on an inheritable disease to your baby.
Birth control (contraception, pregnancy prevention) - Birth control is the practice of preventing or reducing the probability of pregnancy without abstaining from sexual intercourse; the term is also sometimes used to include abortion.
Male condoms - Condoms are thin barriers made of latex, plastic, or natural membranes. The male condom fits over a man's penis. The female condom fits inside a woman's vagina.
Female condoms - The female condom is a polyurethane sheath or pouch about 17 cm (6.5 inches) in length. It is worn by a woman during sex.
Diaphragm - A diaphragm is a rubber disc a woman places into her vagina. The diaphragm blocks a man's semen from entering the cervix (the opening to the womb).
Cervical cap - The cervical cap is a small latex cup that a woman inserts into her vagina before sexual intercourse. The cervical cap fits snugly over the woman's cervix.
Birth control pills - The birth control pill is a small, usually white, tablet that is taken orally (by mouth). The pill usually comes in a packet that has days marked off for a cycle lasting about a month.
Norplant - Contraceptive implants (Norplant?) are six match stick size implants inserted into the upper arm. Norplant is a form of progestin that is placed under the skin.
Depo Provera - Depo Provera is a hormone, much like the progesterone a woman produces during the last two weeks of each monthly cycle. Depo-Provera or progesterone stops the woman's ovaries from releasing an egg.
Spermicides - Spermicides are chemicals that make the sperm unable to function. Spermicide can be used alone or with other birth control methods to reduce the risk of pregnancy.
Emergency contraceptive pill - Emergency contraception is the use of certain methods after unprotected intercourse to prevent pregnancy.
Natural family planning - Natural family planning is defined as the understanding and use of the natural phases of fertility and infertility by a couple in order to either achieve or avoid pregnancy.
Intrauterine device (IUD) - An intrauterine device (intra meaning within, and uterine meaning of the uterus), is a birth control device also known as an IUD or a coil.
Birth control patch - The birth control patch is a thin plastic patch (1 3/4 inch square) placed directly on the skin of the woman. It is a hormonal method of contraception obtained by prescription.
Sterilization (vasectomy) - Sterilization is a surgical technique leaving a male or female unable to procreate. It is a method of birth control.
Fertility awareness method (FAM) - Fertility awareness is a means of understanding a woman's reproductive cycle by observing and writing down fertility signs.
Abstinence - Periodic abstinence is a way that sexually active women prevent pregnancy by becoming familiar with their fertility patterns and abstaining from vaginal intercourse on the days they think they could become pregnant.
Pre-eclampsia, eclampsia - Pre-eclampsia is a condition which only occurs during pregnancy. It causes high blood pressure, protein leaks from the kidneys, and other symptoms may develop.
HELLP Syndrome - The HELLP syndrome is a complication of pregnancy featuring a combination of abnormal conditions including emolysis, elevated liver enzymes, and low platelet count.
Intrauterine growth restriction - Intrauterine growth restriction (IUGR) refers to the condition in which a foetus is unable to grow to its genetically determined potential size to a degree that may affect the health of the foetus.
Premature birth - Premature birth is defined medically as a birth occurring earlier than 37 weeks. Infants born prematurely have an increased risk of death in the first year of life.
Stillbirth - Stillbirth refers to the death of a baby after 24 weeks of pregnancy but before birth. A pregnancy that ends before the twentieth week is called a miscarriage rather than a stillbirth.
Caesarean section - A Caesarean section (Cesarean section AE), is a surgical procedure to deliver one or more babies through an incision in the mother's abdomen and uterus.
Preterm labor - Preterm labor, or premature labor, is when the uterus (womb) contracts and the cervix opens earlier than normal.
Rh incompatibility - Rh incompatibility is a condition that occurs when the mother of a fetus or newborn has Rh-negative blood type and the fetus or newborn has Rh-positive blood.
Ectopic pregnancy - An ectopic pregnancy is one in which the fertilized ovum is implanted in any tissue other than the uterine wall.
Pregnancy diabetes (gestational diabetes) - Gestational diabetes is a condition in which the glucose level is elevated and other diabetic symptoms appear during pregnancy in a woman who has not previously been diagnosed with diabetes.
Group B strep - Group B streptococcus (group B strep) is a type of bacteria that causes infection among newborns, pregnant women or women after childbirth.
Morning sickness (NVP) - Morning sickness, also called nausea and vomiting of pregnancy (NVP), affects between 50 and 85 percent of all pregnant women.
Hyperemesis gravidarum - Hyperemesis gravidarum means excessive vomiting during pregnancy. The severe vomiting associated with hyperemesis gravidarum requires medical attention.
Miscarriage (spontaneous abortion) - Miscarriage is the term used for a pregnancy that ends on it's own, within the first 20 weeks of gestation.
Postpartum hemorrhage - Postpartum bleeding (severe postpartum bleeding) is the loss of more than a pint of blood within the first 24 hours after delivering a baby.
Pregnancy-induced hypertension - Pregnancy-induced hypertension (also referred to as toxemia, preeclampsia and eclampsia) is a condition that may develop during the second half of a woman's pregnancy.
Pica - Pica is a pattern of eating non-nutritive substances (such as dirt or paper), lasting for at least one month.