We found a positive association of maternal height with pregnancy length per se, and the effect was stronger when EDD was estimated by ultrasound than by LMP. Women with shorter stature had lower risk of post-term deliveries, and when EDD was based on ultrasound, they also had higher risk of preterm births. Paternal height and common cardiovascular risk factors of the father showed no association with length of pregnancy or with the risk of pre-and post-term births.
A Norwegian study among women with low risk pregnancies, spontaneous start of delivery, and EDD estimated by LMP found no association of maternal height with length of pregnancy
. However, the authors of a Swedish study among 952 630 unselected pregnant women whose gestational length was estimated by ultrasound in early second trimester, reported that unadjusted mean gestational length was 2 days shorter in mothers of short stature (< 160 cm) compared to those who were taller than 160 cm
We assessed paternal height and levels of paternal cardiovascular risk factors such as blood pressure, BMI, serum glucose and lipids, in relation to pregnancy length and risk of pre- and post-term birth. In contrast to previously reported associations between unfavorable cardiovascular risk factors among mothers and pregnancy length
, no such associations were observed for the fathers. Intergenerational studies have suggested that fathers may be of importance in determining pregnancy length in term and post-term pregnancies
[2, 18, 19], but there has been little evidence for a paternal contribution to the risk of preterm birth
[24, 25]. In this study, paternal height was neither associated with pregnancy length nor with the risk of pre- and post-term birth. To our knowledge, these relations have not been reported previously. In line with a recent review
, however, we found a positive association of paternal height with offspring birth weight.
The population based prospective design of the present study makes it unlikely that selection or recall bias can explain our findings. The attendance to HUNT 2 was 71%, and in a follow-up study of 685 (2.5%) non-responders it was concluded that practical reasons such as time constraints and moving out of the county were the main reasons for young people not to attend
. Thus, the participants at fertile age in our study are likely to be representative for the source population. The relatively large sample size and the standardized measurements of height and other clinical measures in HUNT 2 ensure high precision of the effect estimates, and comprehensive information from self-administered questionnaires provides access to a range of possible confounders. By combining data from the HUNT 2 Study and the MBR it was possible to control for metabolic factors and other known risk factors on an individual basis. A potential limitation in this study is that smoking status was sampled before rather than during pregnancy. This was due to lack of registration of smoking status in MBR until 1999. We performed sensitivity analyses restricted to pregnancies with available information on smoking during pregnancy from 1999 to 2005, and the estimates did not differ substantially from the main results.
The MBR in Norway is a nationwide registry that includes information about virtually all births that have occurred in the country since 1967. Almost all pregnant women in Norway receive antenatal care, and hospital deliveries are free of charge, which minimizes any potential selection bias
. EDD was estimated by two different methods for most of the women (ultrasound and last menstrual period), and the use of both methods was standardized throughout the study period. The internal validity of our results is regarded as good. Generalization of the results to other populations must still be done with caution, since the population under study was rather homogenous with less than 3% of Caucasian women.
The LMP method is limited by inaccurate maternal recall, uncertain time of conception and implantation, irregular menses/oligomenorrea and pre-pregnancy use of hormonal contraceptives. If shorter women have higher risk of hormonal disturbances with delayed ovulation, this could have biased our findings by underestimating EDD in LMP-based analyses. Adjustment for menstrual disturbances in the statistical analyses did not, however, substantially alter the results.
It is generally agreed that ultrasound biometry in early second trimester gives a more accurate prediction of date of delivery (EDD) and reduces the rate of post-term deliveries compared to LMP dating
. However, the ultrasound method is based on the assumption that the size of all fetuses is similar at a given gestational age during the first half of pregnancy, whereas several studies suggest that fetal size (BPD) may differ substantially during the first half of pregnancy according to fetal sex, fetal growth restriction, and maternal smoking
[28–30]. If fetal size (BPD) in early second trimester also differ according to maternal height, ultrasound dating may induce a biased estimate of EDD
. Femur length of the fetus at 18–19 gestational weeks has been reported to correlate with maternal height,
maternal height is a known determinant of offspring birth weight
[5, 33, 34],and fetal size in early second trimester is positively associated with offspring birth weight
[35, 36]. Thus, it is not unlikely that ultrasound in 17–19 weeks of gestation may underestimate the true gestational age of a short woman and shift the EDD to a later date due to her smaller than average sized fetus, and vice versa for taller women. As a result, shorter women may have more severe post-term pregnancies than taller women and may therefore be at higher risk of adverse perinatal outcomes as well
. Taller women, on the other hand, may risk labor inductions on false post-term indications. A large population-based study reported that the replacement of LMP-dating with second trimester ultrasound dating in Sweden resulted in an increased risk of post- term perinatal morbidity and mortality for female fetuses
. The smaller size of female fetuses compared to males at time of ultrasound measurement most likely resulted in an underestimation of the true gestational age and more severely post term pregnancies among mothers of female fetuses. Whether the rate of post-term adverse perinatal outcomes may differ between short and tall women is not known, and unfortunately we did not have sufficient analytical power to investigate this in our data.
The observation that small fetuses grow slower and have longer pregnancies than average-sized fetuses, and vice versa for large fetuses, applies to low risk/non-pathological pregnancies for humans, and are also observed for some other mammals
[2, 4, 38, 39]. The opposite is documented for pathological pregnancies; i.e. women with slow intrauterine fetal growth have increased risk of both spontaneous and iatrogenic preterm births compared to other women
[40, 41]. Since maternal height is a predictor of offspring birth size
[33, 34] and fetal growth may influence pregnancy length, fetal growth could be an intermediate factor for the association of maternal height with gestational length. We did not have access to data on serial ultrasound measurements of fetal size to assess fetal growth. Offspring birth weight could serve as an indicator of fetal growth. However, birth weight was not regarded as a confounder, but as an intermediate factor or a common consequence of maternal height and gestational length. Thus, to avoid introducing a bias to the results, we chose not to adjust the analyses for birth weight. As an alternative approach, we separately assessed associations of parental heights with birth weight z-scores, and found that paternal height was positively associated with offspring birth weight. The association of paternal height with birth weight was of similar strength to that of maternal height, and contrasted the finding of no association between paternal height and pregnancy length. This different effect of parental height may suggest that gestational length and offspring birth weight are determined by different parental factors, and that the positive association of maternal height with gestational length cannot solely be explained by fetal growth.
Short women are at increased risk of cardiovascular disease compared to tall women, and length of pregnancy tends to be shorter in women who are at increased cardiovascular risk
[14, 17]. According to the fetal origins hypothesis, poor nutritional conditions in utero may program both slow intrauterine growth and increased risk of later cardiovascular disease
. If the short stature of the woman has an intrauterine origin, their higher cardiovascular risk may also have originated in utero, and could possibly explain the observed association of short maternal height with short gestational length. However, adjustment for maternal cardiovascular risk factors did not change the effect estimates in the present study, and is therefore an unlikely explanation of the results. Possible effect modification of cardiovascular risk factors was also assessed, but we found no evidence of any interaction. Similarly, unfavorable socioeconomic conditions of the mother could be a common cause for short maternal stature and short pregnancy length, but adjustment for socioeconomic measures did not influence the results.
In line with other research, our descriptive data suggest that shorter women experience pregnancy complications more frequently than taller women, including SGA offspring, and acute and elective Caesarian section
[5, 7]. Preeclampsia, stillbirth and perinatal deaths have also been reported to be associated with short stature of the mother
[5, 8, 10]. After excluding induced births (by medication or by caesarian sections), and preeclampsia, SGA and stillbirths, the associations between maternal height and length of pregnancy became weaker in our study. This indicates that some of the association between maternal height and gestational length may be explained by a higher incidence of pregnancy complications and caesarian sections before labour among shorter women than among taller. The higher frequency of elective caesarian sections among shorter women may further reflect their higher risk of previous complications, such as previous caesarian section and/or traumatic labour experience
We cannot rule out that the observed association of maternal height with gestational length may have some biological basis. Blacks and Asians have shorter average gestational length and higher risk of preterm birth than white Americans and Europeans, and teenage mothers have shorter length of pregnancy and higher risk of preterm birth compared to adult mothers
[44, 45]. A smaller or more constricted female pelvis of teenage and Asian mothers has been suggested to facilitate shorter duration of pregnancy to minimize complications from cephalopelvic disproportion. In evolutionary terms similar mechanisms may explain that shorter women benefit from shorter duration of pregnancy
. The population in the present study is ethnically fairly homogenous and without teenage pregnancies