Pregnancy

Pregnancy

Human Pregnancy is the carrying of one or more offspring, known as a fetus or embryo, in the womb of a woman. In a pregnancy, there can be multiple gestations, as in the case of twins or triplets. 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.
Human pregnancy is the most studied of all mammalian pregnancies.

Terminology

One scientific term for the state of pregnancy is gravidity (adjective "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.

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 in spontaneous 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 several steps, and the interruption of any of them can lead to failure. Through fertilization, 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 beginning of the process, the sperm undergoes a series of changes. As freshly ejaculated sperm is unable or poorly able to fertilize, it must undergo capacitation in the female's reproductive tract over several hours. This increases its motility and destabilizes its membrane, preparing it for the acrosome reaction, the enzymatic penetration of the egg's tough membrane, the zona pellucida, which surrounds the oocyte.

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. 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 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.
There is a standard deviation of 8-9 days surrounding due dates calculated with even the most accurate methods. This means that fewer than 5% of births occur at exactly 40 weeks; 50% of births are within a week of this duration, and about 80% are 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.
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. Therefore, in an otherwise uncomplicated pregnancy, obstetricians usually prefer to induce labour at some stage between 41 and 42 weeks.
Birth before 39 weeks, even if considered "at term", increases the risk of complications and premature death, from factors including under-developed lungs, infection due to under-developed immune system, problems feeding due to under-developed brain, and jaundice from under-developed liver. Some hospitals in the United States noticed an significant increase in neonatal intensive care unit patients when women schedule deliveries for convenience, and are taking steps to reduce induction for non-medical reasons. Complications for Caesarean section are more common than for live births.
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.
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 fetal 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.[citation needed]

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