Pregnancy

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 surgical field that studies and treats pregnancy. Midwifery is the non-surgical field that cares for pregnant women.
Childbirth usually occurs about 38 weeks from fertilization (conception), i.e., approximately 40 weeks from the the last normal menstrual period (LNMP) in humans. The date of delivery is considered normal medically if it falls within two weeks of the calculated date. The calculation of this date involves the assumption of a regular 28-day period. Thus, pregnancy lasts about nine months, although the exact definition of the English word “pregnancy” is a subject of controversy.

Terminology
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, a woman who is not currently pregnant or who has never been pregnant is referred to as a "nulligravida", and in subsequent pregnancies as "multigravida" or "multiparous".[2] 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. Women who have never carried a pregnancy achieving more than 20 weeks of gestation age are referred to as "nulliparous".[3]
The term embryo is used to describe the developing offspring during the first eight weeks following conception, and the term foetus 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 foetus can be more easily monitored and diagnosed. The beginning of the third trimester often approximates the point of viability, or the ability of the foetus to survive, with or without medical help, outside of the uterus.[6]