Pregnancy (latin graviditas) is the carrying of one or more offspring, known as a fetus or embryo, inside the uterus of a female human. In a pregnancy, there can be multiple gestations, as in the case of twins or triplets. Human pregnancy is the most studied of all mammalian pregnancies. Obstetrics is the medical field that studies and treats pregnant patients.
Childbirth usually occurs about 38 weeks from fertilization, i.e., approximately 40 weeks from the start of the last menstruation. Thus, pregnancy lasts about nine months, although the exact definition of the English word “pregnancy” is a subject of controversy.
One scientific term for the state of pregnancy is gravid, and a pregnant female is sometimes referred to as a gravida.[1] Both words are rarely used in common speech. Similarly, the term "parity" (abbreviated as "para") is used for the number of previous successful live births. Medically, women who have never been pregnant are referred to as "nulliparous" ("gravida 0, para 0"),[2], during a first pregnancy as a "primigravida" ("gravida 1, para 0") and in subsequent pregnancies as "multigravida" or "multiparous".[3] Hence during a second pregnancy a woman would be described as "gravida 2, para 1" and upon delivery as "gravida 2, para 2". Incomplete pregnancies of abortions, miscarriages or stillbirths account for parity values being less than the gravida number, whereas a multiple birth will increase the parity value.
The term embryo is used to describe the developing offspring during the initial weeks, and the term fetus is used from about two months of development until birth.[4][5]
In many societies' medical and legal definitions, human pregnancy is somewhat arbitrarily divided into three trimester periods, as a means to simplify reference to the different stages of prenatal development. The first trimester carries the highest risk of miscarriage (natural death of embryo or fetus). During the second trimester, the development of the fetus can be more easily monitored and diagnosed. The beginning of the third trimester often approximates the point of viability, or the ability of the fetus to survive, with or without medical help, outside of the uterus
Characteristics
Pregnancy occurs as the result of the female gamete or oocyte (egg) being penetrated by the male gamete spermatozoon in a process referred to, in medicine, as "fertilization", or more commonly known as "conception". The fusion of male and female gametes usually occurs through the act of sexual intercourse or, very rarely, other non-penetrative sexual activity. However, the advent of artificial insemination has also made achieving pregnancy possible in such cases where sexual intercourse is not potentially fertile (through choice or male/female infertility).
Though pregnancy begins at implantation, it is more convenient to date from the first day of a woman's last menstrual period (acronym = LMP), or from the date of conception (if known). Starting from one of these dates, the expected date of delivery (acronym = EDD) can be calculated. Counting from the LMP, pregnancy usually lasts between 37 and 42 weeks, with the EDD at 40 weeks, 38 weeks after conception. 40 weeks is a little more than nine months and six days, which forms the basis of Naegele's rule for estimating date of delivery.
Pregnancy is considered 'at term' when gestation attains 37 complete weeks but is less than 42 (between 259 and 294 days since LMP). Events before completion of 37 weeks (259 days) are considered pre-term; from week 42 (294 days) events are considered post-term.When a pregnancy exceeds 42 weeks (294 days), the risk of complications for mother and fetus increases significantly.As such, obstetricians usually prefer to induce labour, in an uncomplicated pregnancy, at some stage between 41 and 42 weeks.
Recent medical literature prefers the terminology pre-term and post-term to premature and post-mature. Pre-term and post-term are unambiguously defined as above, whereas premature and postmature have historical meaning and relate more to the infant's size and state of development rather than to the stage of pregnancy.
Though these are the averages, the actual length of pregnancy depends on various factors. For example, the first pregnancy tends to last longer than subsequent pregnancies. Fewer 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.
Accurate dating of pregnancy 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 fetus is perceived to be overdue. Due dates are only a rough estimate, and the process of accurately dating a pregnancy using the LMP method is complicated by the fact that not all women have 28 day menstrual cycles, nor ovulate on the 14th day following their last menstrual period.
A number of medical signs are associated with pregnancy. These signs typically appear, if at all, within the first few weeks after conception. Although not all of these signs are universally present, nor are all of them diagnostic by themselves, taken together they make a presumptive diagnosis of pregnancy. These signs include the presence of human chorionic gonadotropin (hCG) in the blood and urine, missed menstrual period, implantation bleeding that occurs at implantation of the embryo in the uterus during the third or fourth week after last menstrual period, increased basal body temperature sustained for over two weeks after ovulation, Chadwick's sign (darkening of the cervix, vagina, and vulva), Goodell's sign (softening of the vaginal portion of the cervix), Hegar's sign (softening of the Vaginal fornix), and Linea nigra, (darkening of the skin in a vertical line on the abdomen, caused by hyperpigmentation resulting from hormonal changes; it usually appears around the middle of pregnancy).
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