Henriksen T B, Wilcox A J, Hedegaard M, Secher N J
Danish Epidemiology Science Centre, Aarhus University, Denmark.
Epidemiology. 1995 Sep;6(5):533-7. doi: 10.1097/00001648-199509000-00012.
Ultrasound measurement of fetal dimensions is widely used for estimating gestational age. A little-discussed limitation of this method is that variations in fetal size at a given stage of pregnancy are converted to differences in gestational age. Factors that affect pregnancy duration often affect fetal size also. We explore how the effect of such factors may be biased when gestational age is determined by ultrasound. We selected 3,606 women with singleton pregnancies (1989-1991) who had an early ultrasound measurement of fetal biparietal diameter (BPD) and a good-quality history of last menstrual period (LMP). Using the two measures of gestational age, we estimated risk of preterm and postterm delivery for female babies vs males and for smoking women vs nonsmoking women. There was a 13% excess of preterm delivery among female babies when gestational age was determined by ultrasound, but no excess when gestational age was defined by LMP. For postterm delivery, female babies had a 19% lower risk with ultrasound-defined age, but no deficit with LMP-defined age. We found a similar bias with ultrasound in the analysis of maternal smoking. Thus, factors that reduce fetal size inflate the risk of preterm delivery and deflate the risk of postterm delivery when gestational age is based on ultrasound measurement of the fetus. This bias can distort the relative risk of preterm or postterm delivery by 10-20%.
超声测量胎儿大小被广泛用于估算孕周。这种方法一个较少被讨论的局限性在于，怀孕特定阶段胎儿大小的差异会被转化为孕周的差异。影响孕期时长的因素通常也会影响胎儿大小。我们探讨了在通过超声确定孕周时，这些因素的影响可能会如何产生偏差。我们选取了3606名单胎妊娠女性（1989 - 1991年），她们在孕早期进行了胎儿双顶径（BPD）的超声测量，并且有质量良好的末次月经（LMP）记录。利用这两种孕周测量方法，我们估算了女婴与男婴以及吸烟女性与非吸烟女性早产和过期产的风险。当通过超声确定孕周时，女婴的早产风险高出13%，但当通过末次月经确定孕周时则没有这种情况。对于过期产，以超声确定的孕周计算，女婴的风险低19%，但以末次月经确定的孕周计算则没有差异。在对孕妇吸烟情况的分析中，我们发现超声测量也存在类似的偏差。因此，当孕周基于胎儿的超声测量时，那些减小胎儿大小的因素会夸大早产风险，同时降低过期产风险。这种偏差会使早产或过期产的相对风险扭曲10% - 20%。
