新华社伦敦8月7日电俗话说“怀胎十月”，然而由于人类的个体差异等原因，这并不是一个普遍准确的预产期。一项最新研究显示，女性孕期时间彼此之间的正常差异可达5周。
英国《人类的繁衍》杂志7日刊载一份美国研究报告称，女性的预产期一般被定为其最后一次月经后第280天，但其实仅有4％的孕妇会在当天生产，在预产期前后10天内生产的约占七成。
美国国家环境卫生科学研究所对125名怀孕女性进行了跟踪调查，研究人员利用新技术对这些女性受孕的确切时间及分娩时间进行分析后发现，她们的怀胎时间从35周到40.5周不等。这项研究排除了早产等影响孕期时间的状况，受调查的女性都是正常生产。
这项研究还发现，孕期时间与孕妇一系列自身因素相关，比如年龄较大的孕妇孕期较长，自己出生时体重较大的女性往往会孕期长，而前一次怀孕时孕期长的，再次怀孕仍会有较长的孕期。
研究人员说，此前人们就知道女性的孕期有个体差异，本次研究为分析这种差异提供了具体的数据支撑，结果显示正常孕期差异可达5周之多。研究人员指出，医生在为孕妇估算预产期的时候，应该考虑个体差异的影响。（生物谷 Bioon.com）
生物谷推荐的英文摘要
Human Reproduction doi: 10.1093/humrep/det297
Length of human pregnancy and contributors to its natural variation
A.M. Jukic, D.D. Baird, C.R. Weinberg, D.R. McConnaughey andA.J. Wilcox.
STUDY QUESTION How variable is the length of human pregnancy, and are early hormonal events related to gestational length?
SUMMARY ANSWER Among natural conceptions where the date of conception (ovulation) is known, the variation in pregnancy length spanned 37 days, even after excluding women with complications or preterm births.
WHAT IS KNOWN ALREADY Previous studies of length of gestation have either estimated gestational age by last menstrual period (LMP) or ultrasound (both imperfect measures) or included pregnancies conceived through assisted reproductive technology.
STUDY DESIGN, SIZE, DURATION The Early Pregnancy Study was a prospective cohort study (1982–85) that followed 130 singleton pregnancies from unassisted conception to birth, with detailed hormonal measurements through the conception cycle; 125 of these pregnancies were included in this analysis.
PARTICIPANTS/MATERIALS, SETTING, METHODS We calculated the length of gestation beginning at conception (ovulation) in 125 naturally conceived, singleton live births. Ovulation, implantation and corpus luteum (CL) rescue pattern were identified with urinary hormone measurements. We accounted for events that artificially shorten the natural length of gestation (Cesarean delivery or labor induction, i.e. ‘censoring’) using Kaplan–Meier curves and proportional hazards models. We examined hormonal and other factors in relation to length of gestation. We did not have ultrasound information to compare with our gold standard measure.
MAIN RESULTS AND THE ROLE OF CHANCE The median time from ovulation to birth was 268 days (38 weeks, 2 days). Even after excluding six preterm births, the gestational length range was 37 days. The coefficient of variation was higher when measured by LMP (4.9%) than by ovulation (3.7%), reflecting the variability of time of ovulation. Conceptions that took longer to implant also took longer from implantation to delivery (P = 0.02). CL rescue pattern (reflecting ovarian response to implantation) was predictive (P = 0.006): pregnancies with a rapid progesterone rise were longer than those with delayed rise (a 12-day difference in the median gestational length). Mothers with longer gestations were older (P = 0.02), had longer pregnancies in other births (P < 0.0001) and were heavier at birth (P = 0.01). We did not see an association between the length of gestation and several factors that have been associated with gestational length in previous studies: body mass index, alcohol intake, parity or offspring sex.
LIMITATIONS, REASONS FOR CAUTION The sample size was small and some exposures were rare, reducing power to detect weak associations.
WIDER IMPLICATIONS OF THE FINDINGS Human gestational length varies considerably even when measured exactly (from ovulation). An individual woman's deliveries tend to occur at similar gestational ages. Events in the first 2 weeks after conception are predictive of subsequent pregnancy length, and may suggest pathways underlying the timing of delivery.
STUDY FUNDING/COMPETING INTEREST This research was supported by the Intramural Research Program of the NIH, National Institute of Environmental Health Sciences. None of the authors has any conflict of interest to declare.
