The expected date of delivery (EDD) is 40 weeks counting from the last menstrual period (LMP) and birth usually occurs between 37 and 42 weeks,[11] The actual pregnancy duration is typically 38 weeks after conception. Though pregnancy begins at conception, 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. 40 weeks is nine months and six days, which forms the basis of Naegele's rule for estimating date of delivery. More accurate and sophisticated algorithms take 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 pre-term; from week 42 (294 days) events are considered post-term.[12] When a pregnancy exceeds 42 weeks (294 days), the risk of complications for woman and fetus increases significantly.[11][13] As such, obstetricians usually prefer to induce labour, in an uncomplicated pregnancy, at some stage between 41 and 42 weeks.[14][15]
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.[16][17]
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 two weeks.[18] It is much more useful, 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 relentlessly as medical revolution continues to unfold. Whereas it used to be 28 weeks, this has been brought back to as early as 23 weeks [22 weeks in a few countries]. Unfortunately, there has been a profound increase in morbidity and mortality associated with the increased survival to the extent it has led some to question the ethics and morality of resuscitating at the edge of viability.
Childbirth
Childbirth is the process whereby an infant is born. It is considered by many to be the beginning of a person'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.
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