Fetal Size in the Second Trimester is Associated With the Duration of Pregnancy, Small Fetuses Having Longer Pregnancies

Synnøve L. Johnsen; Tom Wilsgaard; Svein Rasmussen; Mark A. Hanson; Keith M. Godfrey; Torvid Kiserud


BMC Pregnancy Childbirth 

In This Article

Abstract and Background


Background: Conventionally, the pregnancy duration is accepted to be 280-282 days. Fetuses determined by ultrasound biometry to be small in early pregnancy, have an increased risk of premature birth. We speculate that the higher rate of preterm delivery in such small fetuses represents a pathological outcome not applicable to physiological pregnancies. Here we test the hypothesis that in low-risk pregnancies fetal growth (expressed by fetal size in the second trimester) is itself a determinant for pregnancy duration with the slower growing fetuses having a longer pregnancy.
Methods: We analysed duration of gestation data for 541 women who had a spontaneous delivery having previously been recruited to a cross-sectional study of 650 low-risk pregnancies. All had a regular menses and a known date of their last menstrual period (LMP). Subjects were examined using ultrasound to determine fetal head circumference (HC), abdominal circumference (AC) and femur length (FL) at 10-24 weeks of gestation. Length of the pregnancy was calculated from LMP, and birth weights were noted. The effect of fetal size at 10-24 weeks of gestation on pregnancy duration was assessed also when adjusting for the difference between LMP and ultrasound based fetal age.
Results: Small fetuses (z-score -2.5) at second trimester ultrasound scan had lower birth weights (p < 0.0001) and longer duration of pregnancy (p < 0.0001) than large fetuses (z-score +2.5): 289.6 days (95%CI 288.0 to 291.1) vs. 276.1 (95%CI 273.6 to 278.4) for HC, 289.0 days (95%CI 287.4 to 290.6) vs. 276.9 days (95%CI 274.4 to 279.2) for AC and 288.3 vs. 277.9 days (95%CI 275.6 to 280.1) for FL. Controlling for the difference between LMP and ultrasound dating (using HC measurement), the effect of fetal size on pregnancy length was reduced to half but was still present for AC and FL (comparing z-score -2.5 with +2.5, 286.6 vs. 280.2 days, p = 0.004, and 286.0 vs. 280.9, p = 0.008, respectively).
Conclusion: Fetal size in the second trimester is a determinant of birth weight and pregnancy duration, small fetuses having lower birth weights and longer pregnancies (up to 13 days compared with large fetuses). Our results support a concept of individually assigned pregnancy duration according to growth rates rather than imposing a standard of 280-282 days on all pregnancies.


It was Nägele and his contemporaries who first suggested counting 40 weeks from the first day of the last menstrual period (LMP) to predict the day of confinement.[1] Subsequently, WHO has also defined the normal length of pregnancy to be 40 weeks (280 days),[2] but studies of population-based birth registries suggest a longer pregnancy duration based on LMP (mean 281-283.6 days).[3] A problem with the LMP-method is that 45-68% of women have irregular periods or uncertain information of their LMP.[4,5] Moreover, the fertile window occurs over a range of days in the menstrual cycle.[6] Ultrasound dating was thought to overcome some of these problems by using fetal size to determine gestational age and thus to predict day of confinement independently of LMP. Based on fetal biparietal diameter (BPD) in the second trimester, pregnancy duration is calculated to be somewhat shorter (mean 280.6 days)[3] than previously thought. Today ultrasound dating has spread to common use and has had the clinically desirable effect of reducing the number of inductions of labour for presumed post-term pregnancies.[7]

While ultrasound dating is useful for those women with uncertain LMP, it is less obvious that this is also valid for pregnancies with reliable information of a regular LMP. Even in this group, ultrasound dating does, however, predict day of confinement more precisely than LMP.[8,9,10] As a consequence ultrasound dating has been recommended as the preferred dating method,[7] although this view has repeatedly been disputed.[11] The reason is that charts for ultrasound dating are based on fetal biometry in pregnancies with certain and regular LMP in the first place. It therefore seems unlikely that the ultrasound method could better predict day of confinement than the LMP itself, unless the ultrasound method also includes a factor that is not yet accounted for.

We hypothesize that, in addition to LMP, fetal growth (reflected in fetal size) might be such a determining factor for pregnancy duration. The aim of the present study was therefore to assess the effect of second trimester fetal size on the duration of pregnancy and the influence of ultrasound dating.


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