Pregnancy is the carrying of one or more offspring, known as a fetus or embryo, inside the womb of a female. 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. Childbirth usually occurs about 38 weeks after conception; in women who have a menstrual cycle length of four weeks, this is approximately 40 weeks from the last normal menstrual period (LNMP). The World Health Organization defines normal term for delivery as between 37 weeks and 42 weeks.

Terminology

One scientific term for the state of pregnancy is gravid, and a pregnant female is sometimes referred to as a gravida. Neither word is 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 has never been pregnant is referred to as a "nulligravida", a woman who is (or has been only) pregnant for the first time as a "primigravida", and a woman in subsequent pregnancies as a multigravida or "multiparous".Hence, during a second pregnancy a woman would be described as "gravida 2, para 1" and upon live delivery as "gravida 2, para 2". An in-progress pregnancy, as well as 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".

The term embryo is used to describe the developing offspring during the first 8 weeks following conception, and the term fetus is used from about 2 months of development until birth.

In many societies' medical or 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.

Progression

Stages in prenatal development, with weeks and months numbered from last menstrual period.

Initiation

The initial stages of human embryogenesis.

Although pregnancy begins with implantation, the process leading to pregnancy occurs earlier as the result of the female gamete, or oocyte, merging with the male gamete,spermatozoon. In medicine this process is referred to as fertilization; in lay terms, it is more commonly known as "conception". After the point of fertilization, the fused product of the female and male gamete is referred to as a zygote or fertilized egg. The fusion of male and female gametes usually occurs following the act of sexual intercourse, resulting inspontaneous pregnancy. However, the advent of artificial insemination and in vitro fertilisation have also made achieving pregnancy possible in cases where sexual intercourse does not result in fertilization (e.g., through choice or male/female infertility).

The process of fertilization occurs in more than a single step, and the interruption of any of these can lead to a failure. Therefore, fertilization is much more than the fusion between the female gamete and male spermatozoon. Through fertilization, the egg and sperm are saved: the egg is activated to begin its developmental program, and the haploid nuclei of the two gametes come together to form the genome of a new diploid organism 

At the very beginning of the process, the sperm undergoes a series of changes which makes pregnancy likely to occur. As freshly ejaculated sperm is unable or poorly able to fertilize, the sperm undergoes the phenomenon called capacitation. It is estimated that during the ejaculation, 300,000,000 sperm are released, from which only 200 reach the oviduct. Capacitation is the process through which the spermatozoon is prepared for the merging with the egg. Capacitation occurs in 5 to 6 hours and it takes place once the sperm reaches the vagina. This is also the process through which the spermatozoon becomes hyperactivated and prepared for the acrosome reaction. In order to be able to fecundate the egg, the sperm must get through the coat surrounding the egg, the so called "zona pellucida". Once the zona pellucida is penetrated, the sperm is able to reach the oocyte. However in order to get through the egg's coat, the sperm undergoes an acrosome reaction that provides it with an enzymatic drill which is able to penetrate the zona pellucida. The acrosome itself is a modified lysosome, situated on the anterior part of the head of the sperm.

Once a sperm penetrates the zona pellucida, it binds to and fuses with the plasma membrane of the oocyte. Binding occurs at the posterior (post-acrosomal) region of the sperm head. After binding occurs, the egg must also undergo a series of metabolic and physical changes which may influence the further development of the zygote. These changes are called in medicine egg activation, mainly because prior to fertilization, the egg is in a latent state.

Methods to assist reproduction also include intracytoplasmic sperm injection, gamete intrafallopian transfer (GIFT), zygote intrafallopian transfer (ZIFT), and embryo cryopreservation (frozen fertilized egg and sperm). These techniques are considered as alternatives to get pregnant by women who have tried unsuccessfully for at least one year. It is estimated that in the United States, more than 6 million adults, or 10% of the adult population, are affected by infertility.

Prenatal period

Prenatal defines the period occurring "around the time of birth", specifically from 22 completed weeks (154 days) of gestation (the time when birth weight is normally 500 g) to 7 completed days after birth.

Legal regulations in different countries include gestation age beginning from 16 to 22 weeks (5 months) before birth.

Postnatal period

The postnatal period begins immediately after the birth of a child and then extends for about six weeks. During this period, the mother's body returns to prepregnancy conditions as far as uterus size and hormone levels are concerned.

Perinatal period

The perinatal period is immediately before to after birth. Depending on the definition, it starts between the 20th to 28th week of gestation and ends between 1 to 4 weeks after birth (the word "perinatal" is a hybrid of the Greek "peri-" meaning 'around or about' and "natal" from the Latin "natus" meaning "birth.").

Duration

The expected date of delivery (EDD) is 40 weeks counting from the first day of the last menstrual period(LMP), and birth usually occurs between 37 and 42 weeks. The actual pregnancy duration is typically 38 weeks after conception. Though pregnancy begins at implantation, it is more convenient to date from the first day of a woman's last menstrual period, or from the date of conception if known. Starting from one of these dates, the expected date of delivery can be calculated using the Naegele's rule for estimating date of delivery. A more accurate and sophisticated algorithm takes into account other variables, such as whether this is the first or subsequent child (i.e., pregnant woman is a primip or a multip, respectively), ethnicity, parental age, length of menstrual cycle, and menstrual regularity.

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 preterm; from week 42 (294 days) events are considered postterm. When a pregnancy exceeds 42 weeks (294 days), the risk of complications for both the woman and the 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 preterm and postterm to premature and postmature.preterm and postterm 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.

Fewer than 5% of births occur on the due date; 50% of births are within a week of the due date, and almost 90% within 2 weeks. It is much more useful and accurate, therefore, to consider a range of due dates, rather than one specific day, with some online due date calculators providing this information.

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. Furthermore, if LMP and ultrasound dating predict different respective due dates, with the latter being later, this might signify slowed fetal growth and therefore require closer review.

The age of viability has been receding because of continued medical progress. Whereas it used to be 28 weeks, it has been brought back to as early as 23, or even 22 weeks in some countries.

Childbirth

Childbirth is the process whereby an infant is born. It is considered by many] to be the beginning of the infant's life, and age is defined relative to this event in most cultures.

A woman is considered to be in labour when she begins experiencing regular 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, while others find that concentrating on the birth helps to quicken labour and lessen the sensations. Most births are successful vaginal births, but sometimes complications arise and a woman may undergo a cesarean section.

During the time immediately after birth, both the mother and the baby are hormonally cued to bond, the mother through the release of oxytocin, a hormone also released during breastfeeding.

Diagnosis

Linea nigra in a woman at 22 weeks pregnant.

The beginning of pregnancy may be detected in a number of different ways, either by a pregnant woman without medical testing, or by using medical tests with or without the assistance of a medical professional.

Most pregnant women experience a number of symptoms,which can signify pregnancy. The symptoms can include nausea and vomiting, excessive tiredness and fatigue,cravings for certain foods that are not normally sought out, and frequent urination particularly during the night.

A number of early 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 ofhuman chorionic gonadotropin (hCG) in the blood and urine, missed menstrual period, implantation bleeding that occurs atimplantation of the embryo in the uterus during the third or fourth week after last menstrual period, increased basal body temperature sustained for over 2 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 uterus isthmus), and pigmentation of linea alba – Linea nigra, (darkening of the skin in a midline of the abdomen, caused by hyperpigmentationresulting from hormonal changes, usually appearing around the middle of pregnancy).

Pregnancy detection can be accomplished using one or more various pregnancy tests, which detect hormones generated by the newly formed placenta. Clinical blood and urine tests can detect pregnancy 12 days after implantation. Blood pregnancy tests are more accurate than urine tests. Home pregnancy tests are urine tests, and normally cannot detect a pregnancy until at least 12 to 15 days after fertilization. A quantitative blood test can determine approximately the date the embryo was conceived.

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.

Despite all the signs, some women may not realize they are pregnant until they are quite far along in their pregnancy. In some cases, a few women have not been aware of their pregnancy until they beginlabour. This can be caused by many factors, including irregular periods (quite common in teenagers), certain medications (not related to conceiving children), and obese women who disregard their weight gain. Others may be in denial of their situation.

An early sonograph can determine the age of the pregnancy fairly accurately. In practice, doctorstypically 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, she has been charting her cycles, or the conception is the result of some types of fertility treatment (such as IUI or IVF), the exact date of fertilization is unknown. Without symptoms such as morning sickness, often the only visible sign of a pregnancy is an interruption of the woman's 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 2 weeks later than the first day of the woman's last menstrual period. 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. The expected date of delivery may also be calculated from sonogram measurement of the fetus. This method is slightly more accurate than methods based on LMP.

Diagnostic criteria are: Women who have menstrual cycles and are 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.