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How Long Does Pregnancy Normally Last?

Human pregnancy lasts about 281 days reckoned from
the first day of last menstrual period

As early as the 1700s it was observed that  birth could be expected  to come about in "the ninth month after the last menstruation by counting one week after the last period and by reckoning the nine months of gestation from that time."  This would make the duration of pregnancy to be about 281 days after the last menstrual period which is supported by modern day studies .
 

 

REFERENCES
1. Baskett TF, Nagele F. Naegele's rule: a reappraisal. BJOG. 2000 Nov;107(11):1433-5.PMID: PMID:11117775
2. Bergsjų P,et al., Duration of human singleton pregnancy. A population-based study. Acta Obstet Gynecol Scand. 1990;69(3):197-207.PMID:2220340
3. Kieler H, et al., The length of human pregnancy as calculated by ultrasonographic measurement of the fetal biparietal diameter.Ultrasound Obstet Gynecol. 1995 Nov;6(5):353-7. Review.PMID:8590208
4. Nguyen TH, et al. Evaluation of ultrasound-estimated date of delivery in 17,450 spontaneous singleton births: do we need to modify Naegele's rule?Ultrasound Obstet Gynecol. 1999 Jul;14(1):23-8.PMID:10461334

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