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Before pregnancy begins, a female oocyte (egg) must join, by spermatozoon in a process referred to in medicine as "fertilisation", or commonly (though perhaps inaccurately) as "conception." This occurs usually through the act of sexual intercourse, in which a man ejaculates inside a woman, thus releasing his sperm. Though pregnancy begins at implantation, it is often convenient to date from the first day of a woman's last menstrual period. This is used to calculate the Estimated Date of Delivery (EDD).
Traditionally (according to Naegele's rule, which is used to calculate the estimated date of delivery, or EDD), a human pregnancy is considered to last approximately 40 weeks (280 days) from the last menstrual period (LMP), or 37 weeks (259 days) from the date of fertilization. However, a pregnancy is considered to have reached term between 37 and 43 weeks from the beginning of the last menstruation. Babies born before the 37 week mark are considered premature, while babies born after the 43 week mark are considered postmature.
According to the Merck Medical Manual, the norm for human pregnancy is approximately 266 days from the date of fertilization. This is 38 weeks, or approximately 8 Gregorian months and 22.5 days, or 9.0 lunar months). Counting from the beginning of the woman's last menstrual cycle, the norm is 40 weeks (the basis for Naegele's rule). Also, less than 10% of births occur on the due date, 50% of births are within a week of the due date, and almost 90% within two weeks. But it is not clear whether this refers to the due date calculated from an early sonograph or from the last menstruation (see further down).
Though these are the averages, the actual length pregnancy depends on various factors. For example, the first pregnancy tends to last longer than subsequent pregnancies.
An accurate date of fertilization is important, because it is used in calculating the results of various prenatal tests (for example, in the triple test). A decision may be made to induce labour if a baby is perceived to be overdue. Due dates are only a rough estimate, and 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. Approximately 3.6% of all women deliver on the due date predicted by LMP, and 4.7% give birth on the day predicted by ultrasound.
The beginning of pregnancy may be detected in a number of ways, including various pregnancy tests which detect hormones generated by the newly-formed placenta. Clinical blood and urine tests can detect pregnancy soon after implantation, which is as early as 6-8 days after fertilization. Home pregnancy tests are personal urine tests, which normally cannot detect a pregnancy until at least 12-15 days after fertilization. Both clinical and home tests can only detect the state of pregnancy, and cannot detect its age.
In the post-implantation phase, the blastocyst secretes a hormone named human chorionic gonadotropin which in turn, stimulates the corpus luteum in the woman's ovary to continue producing progesterone. This 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 woman. Pregnancy tests detect the presence of human chorionic gonadotropin.
An early ultrasound can determine the age of the pregnancy fairly accurately.
In practice, doctors typically express the age of a pregnancy (i.e. an "age" for an embryo) in terms of "menstrual date" based on the first day of a woman's last menstrual period, as the woman reports it. Unless a woman's recent sexual activity has been limited, the exact date of fertilization is unknown. Absent symptoms such as morning sickness, often the only visible sign of a pregnancy is an interruption of her normal monthly menstruation cycle, (i.e. a "late period"). Hence, the "menstrual date" is simply a common educated estimate for the age of a fetus, which is an average of two weeks later than the first day of the woman's last menstrual period. (The margin of error is 0 to 30 days after last menstruation, hence a 14 day average.) The term "conception date" may sometimes be used when that date is more certain, though even medical professionals can be imprecise with their use of the two distinct terms. The due date can be calculated by using Naegele's rule.
There are likewise finer distinctions between the concepts of fertilization and the actual state of pregnancy, which starts with implantation. In a normal pregnancy, the fertilization of the egg usually will have occurred in the Fallopian tubes or in the uterus. (Often, an egg may become fertilized yet fail to become implanted in the uterus.) If the pregnancy is the result of in-vitro fertilization, the fertilization will have occurred in a Petri dish, after which pregnancy begins when one or more zygotes implant after being transferred by a physician in the woman's uterus.
In the context of political debates regarding a proper definition of life, the terminology of pregnancy can be confusing. Because precise assessment of a pregnancy as being at the "embryo" or "fetus" stage is usually undeterminable, the terms (though more clinically precise) are less commonly used than terms like "baby" or "child." The medically and politically neutral term which remains is simply "pregnancy," though this can be problematic as it only refers indirectly to the embryo or fetus. In the context of personal treatment, bedside manner generally dictates that doctors make sparse use of clinical language like "fetus" and "embryo," and instead simply refer to the developing child as a "baby", though this is not medically accurate.
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Timeline of a typical pregnancy
Pregnancy is typically broken into three periods, or trimesters, each of about three months. While there are no hard and fast rules, these distinctions are useful in describing the changes that take place over time.
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Implantation
In medicine, pregnancy is defined as beginning when the developing embryo becomes implanted into the endometrial lining of a woman's uterus. In some cases where complications may have arisen, the fertilized egg might implant itself in the fallopian tubes or the cervix, causing an ectopic pregnancy. Most pregnant women do not have any specific signs or symptoms of implantation, although it is not uncommon to experience light bleeding at implantation. Some women will also experience cramping during their first trimester. This is usually of no concern unless there is spotting or bleeding as well. The outer layers of the embryo grow and form a placenta, for the purpose of receiving essential nutrients through the uterine wall, or endometrium. The umbilical cord in a newborn child consists of the remnants of the connection to the placenta. The developing embryo undergoes tremendous growth and changes during the process of embryonic and fetal development.
Morning sickness afflicts about seventy percent of all pregnant women, typically only in the first trimester.
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Second trimester
Months 4 through 6 of the pregnancy are called the second trimester. Most women feel more energised in this period, and begin to put on weight. The first movement of the fetus, often referred to as "quickening", can be felt, as it begins to form into a recognizable shape. This typically happens by the fourth month. The reproductive organs can be recognized, and can distingush the fetus as male or female.
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Third trimester
Final weight gain takes place, and the fetus begins to move regularly. The mother's belly button may "pop" out due to her growing belly. This period of her pregnancy can be uncomfortable, causing symptoms like weak bladder control and back-ache. Movement of the baby becomes stronger and more frequent and the fetus prepares for viability outside the womb through improved brain, eye, and muscle function.
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Medical aspects of pregnancy
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Diagnostic criteria are: In a woman who has regular menstrual cycles and is sexually active, a period delayed by a few days or weeks is suggestive of pregnancy; elevated B-hcG to around 100,000 mIU/mL by 10 weeks of gestation.
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Birth
Childbirth is the process in which the baby is born. It is considered by many to be the beginning of a person's life, where age is defined relative to this event in most cultures.
A woman is considered to be in labour when she begins experiencing regular painful uterine contractions, accompanied by changes of her cervix — primarily effacement and dilation. While childbirth is widely experienced as painful, some women do report painless labours. Most women are capable of having a normal birth. However, sometimes complications arise and a woman may need to undergo a caesarean section
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Postnatal period
For topics following on from a successful pregnancy and birth, see:
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Medical disorders in pregnancy
Usually, labor and delivery occur without any problems. Serious problems are relatively rare, and most can be anticipated and treated effectively. However, problems sometimes develop suddenly and unexpectedly. Regular visits to a doctor or midwife during pregnancy make anticipation of problems possible and improve the chances of having a healthy baby.
Approximately 4 million births occur in the United States each year. A significant proportion of these are complicated by one or more medical disorders . Two decades ago, many medical disorders were contraindications to pregnancy. Advances in obstetrics, neonatology, obstetric anesthesiology, and medicine have increased the expectation that pregnancy will result in an excellent outcome for both mother and fetus despite most of these conditions.
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Technical
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embryo - conceptus between time of fertilization to 10 weeks of gestation
FASD - Fetal Alcohol Spectrum Disorder, a clinical term for the effects alcohol can have on the developing fetus
fetus - from 10 weeks of gestation to time of birth
Ga Pw-x-y-z - a = number of pregnancies, w = number of term births, x = number of preterm births, y = number of miscarriages, z = number of living children; for example, G4P1-2-1-3 means the woman had a total of 4 pregnancies, of which 1 is of term, 2 are preterm, 1 miscarriage, and 3 total living children (1 term + 2 preterm).
Gestational age - time from last menstrual period (LMP) up to present
gravidity (G) - number of times a woman has been pregnant
infant - time of birth to 1 year of age
parity (P) - number of pregnancies with a birth beyond 20 weeks GA or an infant weighing more than 500 g
preterm infant - delivered between 24-37 weeks
previable infant - delivered prior to 24 weeks
term infant - delivered between 37-42 weeks
first trimester - up to 14 weeks of gestation
second trimester - 14 to 28 weeks of gestation
third trimester - 28th week to delivery
viability - minimum age for fetus survival, ca. third trimester
zygote - from fertilization until second cell division
full term refers to the end of 40 weeks (nine months) from the first day of the mother's last menstrual period — the end of gestation. If a woman gives birth more than three weeks earlier than this, it is classed as a premature birth.
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Colloquial
There are a number of colloquialisms for pregnancy, usually regional. One way of referring to the action of impregnating a woman, used mainly in Canada and some regions of the United States, is "knocking (her) up." The phrase "knocked up" is also a regional euphemism for "pregnant." The words "gone" or "along" may be used to represent gestational time, e.g. "she's really far gone," or "six months along." In the southern U.S., the metaphor of a water well is occasionally used to represent pregnancy (e.g. "drink out of the well," or to become pregnant), and a baby almost ready to be delivered is "on his/her road." Eastern Seaboard slang describes the woman as being "in a fix" or, occasionally, "preggers"; the Southern U.S. equivalent is "in the family way." An alternate term, not slang or colloquial (but slightly archaic) is "with child." "Having a bun in the oven" is another colloquial phrase sometimes used to indicate that a woman is pregnant. In Australia, especially when the pregnancy is unplanned, the woman is said to be "up the duff."*
Pregnant can also mean "having many possibilities or implications", or "filled with" as in Thomas Gray's famous quote, "some heart once pregnant with celestial fire".
The most common reference to pregnancy is "expecting", an obvious term of a woman is expecting a baby/child. Other colloquialisms related to pregnancy are to "drop", "pop", "blow" and "burst" not meant as literally (e.g. "she is about to burst").
Colloquialisms and comparisons of pregnancy exist universally. In France and throughout Europe, phrases translating roughly to "she's full" means her body, the womb, is "full" with a baby inside.
In Korea, China, and Vietnam, the culture measures age starting from conception to the 10th month, thus a newborn baby is "a year old". It's not to debate if it's a living or unborn, but it was present inside the mother before it arrived.
In Mexico and central America, the ancient Mayan calendar of 276 days was said to have originated from the human gestational cycle, or to indicate the world was created as slowly as a baby develops.
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See also
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Reference
Mittendorf R, Williams MA, Berkey CS, Cotter PF. The length of uncomplicated human gestation. Obstet Gynecol 1990;75:929-32. PMID 2342739.
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