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A preterm birth, also known as premature birth, is defined as babies born alive before 37 weeks of pregnancy are completed. [40] There are three types of preterm births: extremely preterm (less than 28 weeks), very preterm (28 to 32 weeks) and moderate to late preterm (32 to 37 weeks). [40]
Group A had 90 fetuses of 9–12 weeks of gestation and the weight varied between 1-14 grams. Group B had 337 fetuses, age 13–16 weeks of gestation with weight variation between 15 and 105 grams whereas the third group had 435 fetuses of 17–20 weeks of gestation with a weight range between 106 and 310 grams.
Normal weight at term delivery is 2,500–4,200 g (5 lb 8 oz – 9 lb 4 oz). [citation needed] SGA is not a synonym of low birth weight, very low birth weight, or extremely low birth weight. For example, with a 35-week gestational age delivery, a weight of 2,250 g (4 lb 15 oz) is appropriate for gestational age but is still low birth weight.
LBW is either caused by preterm birth (that is, a low gestational age at birth, commonly defined as younger than 37 weeks of gestation) or the infant being small for gestational age (that is, a slow prenatal growth rate), or a combination of both.
The fetus is considered full-term between weeks 37 and 40 when it is sufficiently developed for life outside the uterus. [ 14 ] [ 15 ] It may be 48 to 53 cm (19 to 21 in) in length when born. Control of movement is limited at birth, and purposeful voluntary movements continue to develop until puberty .
The growth rate of a fetus is linear up to 37 weeks of gestation, after which it plateaus. [9] The growth rate of an embryo and infant can be reflected as the weight per gestational age , and is often given as the weight put in relation to what would be expected by the gestational age.
A fetus born prior to the 37th week of gestation is considered to be preterm. A preterm baby is likely to be premature and consequently faces increased risk of morbidity and mortality. An estimated due date is given by Naegele's rule. According to the WHO, a preterm birth is defined as "babies born alive before 37 weeks of pregnancy are completed."
In humans, the usual definition of preterm birth is birth before a gestational age of 37 complete weeks. [94] In the normal human fetus, several organ systems mature between 34 and 37 weeks, and the fetus reaches adequate maturity by the end of this period. One of the main organs greatly affected by premature birth is the lungs.