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Pregnancy is the carrying of one or more embryos or fetuses by female mammals, including humans, inside their bodies. In a pregnancy there can be multiple gestations (for example, in the case of twins, or triplets). Human pregnancy is the most studied of all mammalian pregnancies. Human pregnancy lasts approximately 40 weeks between the time of the last menstrual cycle and birth (38 weeks from fertilisation). The medical term for a pregnant woman is "genetalian," just as the medical term for the unborn human is an embryo (early weeks) and then "foetus" (until birth). A woman who is pregnant for the first time is known as a primigravida or gravida 1: a woman who has never been pregnant is known as a gravida 0; similarly, the terms para 0, para 1 and so on are used for the number of times a woman has given birth.

In many societies' medical and legal definitions, human pregnancy is arbitrarily divided into three trimester periods, as a means to simplify reference to the different stages of fetal development. The first trimester period carries the highest risk of miscarriage (natural death of embryo or fetus), while during the second trimester the development of the fetus can start to be monitored and diagnosed. The third trimester marks the beginning of viability, which means the fetus can survive if an early natural or induced birth occurs. Because of the possible viability of developed fetus, cultural and legal definitions of life often consider a fetus in the third trimester to be a distinct living person.

See also Pregnancy terms and definitions

Detection and dating

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 as early as 6-8 days after date of conception. Home pregnancy tests are personal urine tests, which normally can't detect a pregnancy until at least 12-15 days after conception. Both clinical and home tests can only detect the state of pregnancy, and cannot detect the actual date of conception.

In practice, for the purpose of giving a date for a conception (i.e. an "age" for an embryo), doctors typically date the pregnancy by "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 conception or implantation are unknown. And absent any symptoms of morning sickness, the only visible sign of a pregnancy is often 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 conception, (the margin of error considers 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. An unknown date for conception means that in practice the distinction between embryo and fetus is a clinical one only, and not used as to refer to stages of development of a particular pregnancy.

There are likewise finer distinctions between the concepts of fertilization (conception) and the actual state of pregnancy. In a normal pregnancy, the fertilization of the egg usually will have occurred in the Fallopian tubes or in the uterus. (In women with fertility problems, 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 implants 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."

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.

First trimester

Fertilization

Before pregnancy begins, a female oocyte (egg) must be fertilized, by male sperm in a process referred to in medicine as "fertilization," or commonly as "conception." Pregnancy is usually dated as beginning on the first day of a woman's last menstrual period. This date is used to estimate an EDD, or Estimated Date of Delivery.

Traditionally (according to Naegele's Rule, which is used to calculate the estimated date of delivery (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 conception. However, a pregnancy is considered to have reached term between 37 and 43 weeks. Babies born before the 37 week mark are considered premature, while babies born after the 43 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. 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 mothers deliver on the due date predicted by LMP, and only 4.7% give birth on the day predicted by ultrasound.

Implantation

In medicine, pregnancy is defined as beginning when the developing embryo becomes implanted into the endometrial lining of a woman's uterus. The outer layers of the embryo grow and form a placenta, for the purpose of receiving essential nutrients through the uterus wall. The umbilical cord in a newborn child signifies the remnants of the connection to the placenta. The developing baby undergoes tremendous growth and changes during the process of embryonic and fetal development.

Morning sickness aflicts about half of all pregnant women, typically only in the first trimester.

Second trimester

Most women feel more energised in this period, and begin to seriously put on weight. The first movement of the baby, often referred to as "quickening", can be felt, as the baby begins to form into a recognisable shape.

Third trimester

Final weight gain takes place, and the baby begins to move regularly. This can be uncomfortable, causing symptoms like weak bladder control and back-ache.

Medical aspects of pregnancy

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.

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.

Postnatal Period

For topics following on from a successful pregnancy and birth, see:

• Breastfeeding
• Child development

Terms and definitions

Technical

• zygote - from fertilization until second cell division
• embryo - conceptus between time of fertilization to 10 weeks of gestation
• fetus - from 10 weeks of gestation to time of birth
• FASD - Fetal Alcohol Spectrum Disorder, a clinical term for the effects alcohol can have on the developing fetus
• 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
• viability - minimum age for fetus survival, ca. third trimester

• previable infant - delivered prior to 24 weeks
• preterm infant - delivered between 24-37 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 - 28 weeks to delivery

• parity (P) - number of pregnancies with a birth beyond 20 weeks GA or an infant weighing more than 500 g
• 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).

Colloquial

There are a number of colloquialisms for pregnancy, usually regional. The action of impregnating a woman or girl is called 'knocking (her) up' in Canada and some parts of the U.S., and the state of being pregnant 'knocked-up'. The term 'lady-in-waiting', meaning a pregnant mother, is used broadly in the U.S. The word 'gone' or 'along' is used to represent gestational time, e.g. 'she's really far gone' or 'about 6 weeks gone' or 'six months along'. In the southern U.S. the euphemism of a water well is occasionally used to represent pregnancy (e.g. 'drink out of the well', to become pregnant), and a baby almost ready to be delivered is 'on his/her road'. Eastern Seaboard slang describes the mother 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 is 'with child', now restricted mainly to England. 'Having a bun in the oven' is another frequently used phrase to indicate that a woman is pregnant.

See also

• Abortion
• Lamaze
• Obstetrics
• Contraception
• Twin and Multiple birth
• Teenage pregnancy
• Pregnancy discrimination
• Low birth weight paradox
• Pregnancy in science fiction
• Melasma
• Wrongful abortion
• False pregnancy
• Simulated pregnancy

Reference

• Mittendorf R, Williams MA, Berkey CS, Cotter PF. The length of uncomplicated human gestation. Obstet Gynecol 1990;75:929-32. PMID 2342739.

External links

• Home Pregnancy Test and Stages
• BabyCenter has an extensive library of articles on pregnancy and childbirth
• Early pregnancy: Morning sickness, fatigue and other common symptoms, from MayoClinic.com
• Pregnancy Conception Overview FAQs
• The visible embryo
• Normal Pregnancy, Labor, And Delivery (Merck Manual)
• Alan Guttmacher Institute [http://www.agi-usa.org/pubs/tgr/08/2/gr080207.html (2005) The Implications of Defining When a Woman Is Pregnant discussion of the political and legal background.

• Pregnancy