First-trimester maternal serum levels of placental hormones are independent predictors of second-trimester fetal growth parameters

Correlation Between PAPP-A Levels Determined During the First Trimester and Birth Weight at Full-Term

Foetal birth weight is an important determinant of perinatal health. For this reason, various methods have been investigated for estimating this weight during pregnancy. The aim of this study is to evaluate the possible relationship between full-term birth weight and pregnancy-associated plasma protein-A (PAPP-A) levels determined during the first trimester as part of combined screening for aneuploidy carried out in pregnant women. We carried out a single-centre study including pregnant women who were being followed up by the Obstetrics Service Care Units of the XXI de Santiago de Compostela e Barbanza Foundation, who gave birth from March 1, 2015, to March 1, 2017, and who had undergone their first-trimester combined chromosomopathy screening. The sample included a total of 2794 women. We found a significant correlation between MoM PAPP-A and foetal birth weight. When MoM PAPP-A was measured at extremely low levels (< 0.3) during the first trimester, the OR for giving birth to a foetus with weight < p10, adjusting for gestational age and sex, was 2.74. For low levels of MoM PAPP-A (0.3-0.44), the OR was 1.52. With regard to the value of MOM PAPP-A levels as a predictor of foetal macrosomia, a correlation could be observed with elevated levels, although this was not statistically significant. PAPP-A determined during the first trimester acts as a predictor of foetal weight at term as well as for foetal growth disorders.

Biomarkers at 6 weeks' gestation in the prediction of early miscarriage in pregnancy following assisted reproductive technology

INTRODUCTION

Miscarriage is a major concern in early pregnancy among women having conceived with assisted reproductive treatments. This study aimed to examine potential miscarriage-related biophysical and biochemical markers at 6 weeks' gestation among women with confirmed clinical pregnancy following in vitro fertilization (IVF)/embryo transfer (ET) and evaluate the performance of a model combining maternal factors, biophysical and biochemical markers at 6 weeks' gestation in the prediction of first trimester miscarriage among singleton pregnancies following IVF/ET.

MATERIAL AND METHODS

A prospective cohort study was conducted in a teaching hospital between December 2017 and January 2020 including women who conceived through IVF/ET. Maternal mean arterial pressure, ultrasound markers including mean gestational sac diameter, fetal heart activity, crown rump length and mean uterine artery pulsatility index (mUTPI) and biochemical biomarkers including maternal serum soluble fms-like tyrosine kinase-1 (sFlt-1), placental growth factor (PlGF), kisspeptin and glycodelin-A were measured at 6 weeks' gestation. Logistic regression analysis was carried out to determine significant predictors of miscarriage prior to 13 weeks' gestation and performance of screening was estimated by receiver-operating characteristics curve analysis.

RESULTS

Among 169 included pregnancies, 145 (85.8%) pregnancies progressed to beyond 13 weeks' gestation and had live births whereas 24 (14.2%) pregnancies resulted in a miscarriage during the first trimester. In the miscarriage group, compared to the live birth group, maternal age, body mass index, and mean arterial pressure were significantly increased; mean gestational sac diameter, crown rump length, mUTPI, serum sFlt-1, glycodelin-A, and the rate of positive fetal heart activity were significantly decreased, while no significant differences were detected in PlGF and kisspeptin. Significant prediction for miscarriage before 13 weeks' gestation was provided by maternal age, fetal heart activity, mUTPI, and serum glycodelin-A. The combination of maternal age, ultrasound (fetal heart activity and mUTPI), and biochemical (glycodelin-A) markers achieved the highest area under the curve (AUC: 0.918, 95% CI 0.866-0.955), with estimated detection rates of 54.2% and 70.8% for miscarriage before 13 weeks' gestation, at fixed false positive rates of 5% and 10%, respectively.

CONCLUSIONS

A combination of maternal age, fetal heart activity, mUTPI, and serum glycodelin-A at 6 weeks' gestation could effectively identify IVF/ET pregnancies at risk of first trimester miscarriage.

Is the first-trimester combined screening result associated with the phenotype of Down syndrome? A population-based cohort study

OBJECTIVE

To investigate if the Down syndrome phenotype differs according to the result of first-trimester combined screening (FTS).

METHOD

We included all Down syndrome cases diagnosed by karyotype in pregnancy or after birth in Denmark during 2005-2018. We compared screen positive (odds ≥1:300) and screen negative (odds <1:300) cases as well as screen result subgroups with respect to anthropometrics, congenital malformations, childhood diseases, and hospitalization.

RESULTS

Absolute measures of fetal and birth anthropometrics were comparable between groups. A prenatal malformation diagnosis was more prevalent among screen positive than screen negative cases. Analyses suggested that this could reflect a detection bias. Cases with a screen result of 1:2-1:10 had a higher probability of being diagnosed with a malformation prenatally and with severe congenital heart disease (CHD) postnatally compared with a result of 1:11-1:300. Screen positive cases more often had non-severe CHD but less often a non-heart malformation compared with screen negative cases, while proportions of severe CHD were similar in these groups. Data on hospitalizations showed inconsistent results.

CONCLUSION

The 1:300 screening threshold had limited or no value in predicting Down syndrome phenotype severity. In contrast, cases with a screen result between 1:2 and 1:10 may represent a more severe phenotype.

First trimester PAPP-A serum levels and long-term metabolic outcome of mothers and their offspring

Low maternal serum levels of pregnancy associated plasma protein A (PAPP-A) are known to be associated with the development of pregnancy-related complications like small for gestational age infants, intrauterine fetal demise, gestational diabetes and preeclampsia. The study aims to find possible long-term correlations with the development of metabolic and cardiovascular complications in the mothers and their progeny in later life. This is a retrospective cohort study conducted on consecutive unselected women screened for chromosomal anomalies in the first trimester of pregnancy between 2004 and 2010. PAPP-A values as well as clinical data collected at childbirth were considered. A maternal and neonatal follow-up was performed through a telephone interview with the mother during 2015. The body-mass-index and the presence of cardiovascular diseases, dyslipidaemia and diabetes mellitus were evaluated. The analysis included 988 patients. The median time of follow-up was 7 years (IQR 6-9). Lower first trimester maternal blood PAPP-A quartiles were associated with small stature of the offspring (z-score 1st-2nd quartile 0.37 IQR -0.42 and 1.17 vs 3rd-4th quartile 0.67 IQR -0.17 and 1.36, p < 0.05). Furthermore, low first trimester PAPP-A in pregnancy without other gestations following the index one, in Kaplan-Meier analysis was associated to a significant increase of hypoglycemic agents use at 7 and 10 years (respectively 1.12% CI.95 0-2.38% and 5.45% CI.95 0-10.82%) compared to the control group of high first trimester PAPP-A values (0% CI.95 0-0%) (p < 0.05). Low PAPP-A serum levels in the first trimester of pregnancy are associated with short stature in offspring and de-novo development of maternal diabetes mellitus in later life.

Human placental growth hormone in normal and abnormal fetal growth

Human placental growth hormone (PGH), encoded by the growth hormone (GH) variant gene on chromosome 17, is expressed in the syncytiotrophoblast and extravillous cytotrophoblast layers of the human placenta. Its maternal serum levels increase throughout pregnancy, and gradually replaces the pulsatile secreted pituitary GH. PGH is also detectable in cord blood and in the amniotic fluid. This placental-origin hormone stimulates glyconeogenesis, lipolysis and anabolism in maternal organs, and influences fetal growth, placental development and maternal adaptation to pregnancy. The majority of these actions are performed indirectly by regulating maternal insulin-like growth factor-I levels, while the extravillous trophoblast involvement indicates a direct effect on placental development, as it stimulates trophoblast invasiveness and function via a potential combination of autocrine and paracrine mechanisms. The current review focuses on the role of PGH in fetal growth. In addition, the association of PGH alterations in maternal circulation and placental expression in pregnancy complications associated with abnormal fetal growth is briefly reviewed.

Screening antenatal anxiety: Predicting its effect on fetal growth

Context:

Maternal anxiety has an association with low birth weight. However, studies are scarce to determine any association between maternal anxiety and fetal growth.

Aims:

This study aims to determine the effect of maternal anxiety on fetal growth, measured by gestational age-dependent increase in fetal abdominal circumference (AC). The secondary objective is to determine the effect of maternal anxiety on other fetal parameters (biparietal diameter [BPD], head circumference [HC], femur length [FL]).

Settings and Design:

This cross-sectional study was conducted in a tertiary care hospital, Kolkata.

Materials and Methods:

Four hundred and ten pregnant mothers, between 14 and 40 weeks of gestation, were interviewed with socioeconomic and obstetric profile questionnaire and examined for anthropometric profile and presence and severity of pallor. Anxiety was assessed using Generalized Anxiety Disorder-7 (GAD) questionnaire. HC, AC, BPD, and FL were measured by ultrasound biometry.

Analysis Used:

A multivariable logistic regression analysis was done to determine the predictors of small-for-gestational-age (SGA). A robust mediation analysis was done to determine mediating effect of anxiety on gestational age-dependent increase in fetal AC.

Results:

Mild (odds ratio [OR]_Adjusted = 6.23, [2.41, 16.15]) and moderate (OR_Adjusted = 22.42, [5.00, 100.57]) anxiety was significantly associated with SGA fetus. Anxiety increased with the progression of gestation (β_GAD: 0.011 [0.007–0.015]) and it had a negative effect on fetal growth (standardized indirect effect of gestational age-mediated by anxiety on AC: −0.037 [−0.059, −0.022]). Anxiety also attenuated gestational age-dependent increment of HC.

Conclusion:

Mother's anxiety has a gestational age-dependent temporally incremental negative effect on fetal growth and brain development.

Isolated Short Fetal Femur Length in the Second Trimester and the Association with Adverse Perinatal Outcome: Experiences from a Tertiary Referral Center

Objective

To determine the association between isolated mid-trimester short fetal femur length and adverse perinatal outcome.

Methods

This is a retrospective cohort study of patients with singleton gestations routinely assessed by second trimester ultrasound examination during 2006-2013. A fetal isolated short femur was defined as a femur length (FL) below the 5^th percentile in a fetus with an abdominal circumference greater than the 10^th percentile. Cases of aneuploidy, skeletal dysplasia and major anomalies were excluded. Primary outcomes of interest included the risk of small for gestational age neonates, low birth weight and preterm birth (PTB). Secondary outcome parameters were a 5-min Apgar score less than 7 and a neonatal intensive care unit admission. A control group of 200 fetuses with FL ≥ 5th percentile was used to compare primary and secondary outcome parameters within both groups. Chi-square and Student’s t-tests were used where appropriate.

Results

Out of 608 eligible patients with a short FL, 117 met the inclusion criteria. Isolated short FL was associated with an increased risk for small for gestational age (19.7% versus 8.0%, p = 0.002) neonates, low birth weight (23.9% versus 8.5%, p<0.001), PTB (19.7% versus 6.0%, p<0.001) and neonatal intensive care unit admissions (13.7% versus 3.5%, p = 0.001). The incidence of a 5-min Apgar score less than 7 was similar in both groups.

Conclusion

Isolated short FL is associated with a subsequent delivery of small for gestational age and Low birth weight neonates as well as an increased risk for PTB. This information should be considered when counseling patients after mid-trimester isolated short FL is diagnosed.

Reference ranges and determinants of total hCG levels during pregnancy: the Generation R Study

Human chorionic gonadotropin (hCG) is a pregnancy hormone secreted by the placental synctiotrophoblast cell layer that has been linked to fetal growth and various placental, uterine and fetal functions. In order to investigate the effects of hCG on clinical endpoints, knowledge on reference range (RR) methodology and determinants of gestational hCG levels is crucial. Moreover, a better understanding of gestational hCG physiology can improve current screening programs and future clinical management. Serum total hCG levels were determined in 8195 women participating in the Generation R Study. Gestational age specific RRs using ‘ultrasound derived gestational age’ (US RRs) were calculated and compared with ‘last menstrual period derived gestational age’ (LMP RRs) and a model-based RR. We also investigated which pregnancy characteristics were associated with hCG levels. Compared to the US RRs, the LMP RRs were lower, most notably for the median and lower limit levels. No considerable differences were found between RRs calculated in the general population or in uncomplicated pregnancies only. Maternal smoking, BMI, parity, ethnicity, fetal gender, placental weight and hyperemesis gravidarum symptoms were associated with total hCG. We provide gestational RRs for total hCG and show that total hCG values and RR cut-offs during pregnancy vary depending on pregnancy dating methodology. This is likely due to the influence of hCG on embryonic growth, suggesting that ultrasound based pregnancy dating might be less reliable in women with high/low hCG levels. Furthermore, we identify different pregnancy characteristics that influence total hCG levels considerably and should therefore be accounted for in clinical studies.

The effect of infertility and assisted reproduction on first-trimester placental and fetal development

Our investigation of differences in first-trimester placentation and/or fetal biometry in pregnancies conceived spontaneously compared with those conceived through fertility treatment, and comparing less invasive fertility treatment with in vitro fertilization found no statistically significant difference in first-trimester abnormal placentation or fetal growth between pregnancies conceived spontaneously and those conceived through fertility treatment, or between the in vivo and in vitro subgroups.

Low PAPP-A in the first trimester is associated with reduced fetal growth rate prior to gestational week 20

OBJECTIVE

To evaluate the association between maternal pregnancy-associated plasma protein-A (PAPP-A) and fetal growth from the first to the second trimester.

METHODS

A prospective cohort study including 8347 pregnant women attending prenatal care at Aarhus University Hospital were conducted. PAPP-A was measured during 8 to 14 gestational weeks. Fetal growth between the two scans in the first and second trimesters was estimated by (GA(20)- GA(12))/Days(calendar), where GA(12) reflects gestational age in days calculated from crown-rump length at a 12-week scan, GA(20) reflects gestational age in days calculated from biparietal diameter at a 20-week scan, and Days(calendar) reflects the number of calendar days between the two scans.

RESULTS

Fetal growth rate from the first to the second trimester was correlated with PAPP-A, with a regression coefficient of 0.009 (95% CI, 0.007-0.012, P < 0.001). PAPP-A below 0.30 MoM was associated with a fetal growth rate below the tenth centile, with an adjusted OR of 2.05 (95% CI, 1.24-3.38).

CONCLUSION

Low levels of PAPP-A are associated not only with low birth weight at term but also with slower fetal growth prior to 20 weeks of gestation.

Prediction of adverse pregnancy outcomes by combinations of first and second trimester biochemistry markers used in the routine prenatal screening of Down syndrome

OBJECTIVE

To investigate the associations between four defined adverse pregnancy outcomes and levels of first and second trimester maternal serum markers focusing in particular on how well combinations of markers predict these adverse outcomes.

METHODS

This was a retrospective review of associations between first and second trimester serum markers and adverse pregnancy outcomes among 141 698 women who underwent prenatal screening for Down syndrome in Ontario, Canada. Detection rates (DR), false positive rates (FPR), and odds ratios were estimated using both single and combinations of markers for the adverse outcomes defined.

RESULTS

Women with decreased second trimester unconjugated oestriol (uE3), deceased first trimester maternal serum pregnancy-associated plasma protein A (PAPP-A), increased second trimester serum alpha fetoprotein (AFP), or increased second trimester total human chorionic gonadotrophin (hCG) were at greater risk of developing adverse pregnancy outcomes. At a 5% FPR, combinations of these markers predicted at best 33.3% of fetal loss and 31.5% of preterm births (PTB) before 32 weeks of gestation.

CONCLUSION

There are significant associations between the levels of first and second trimester serum markers and adverse obstetric outcomes. However, even combinations of these markers can only predict adverse obstetric outcomes with modest accuracy.

Early rapid growth, early birth: accelerated fetal growth and spontaneous late preterm birth

The past two decades in the United States have seen a 24% rise in spontaneous late preterm delivery (34-36 weeks) of unknown etiology. This study tested the hypothesis that fetal growth was identical prior to spontaneous preterm (n = 221, median gestational age at birth 35.6 weeks) and term (n = 3706) birth among pregnancies followed longitudinally in Santiago, Chile. The hypothesis was not supported: Preterm-delivered fetuses were significantly larger than their term-delivered peers by mid-second trimester in estimated fetal weight, head, limb, and abdominal dimensions, and they followed different growth trajectories. Piecewise regression assessed time-specific differences in growth rates at 4-week intervals from 16 weeks. Estimated fetal weight and abdominal circumference growth rates slowed at 20 weeks among the preterm-delivered, only to match and/or exceed their term-delivered peers at 24-28 weeks. After an abrupt growth rate decline at 28 weeks, fetuses delivered preterm did so at greater population-specific sex and age-adjusted birth weight percentiles than their peers from uncomplicated pregnancies (P < 0.01). Growth rates predicted birth timing: one standard score of estimated fetal weight increased the odds ratio for late preterm birth from 2.8 prior to 23 weeks, to 3.6 (95% confidence interval, 1.82-7.11, P < 0.05) between 23 and 27 weeks. After 27 weeks, increasing size was protective (OR: 0.56, 95% confidence interval, 0.38-0.82, P = 0.003). These data document, for the first time, a distinctive fetal growth pattern across gestation preceding spontaneous late preterm birth, identify the importance of mid-gestation for alterations in fetal growth, and add perspective on human fetal biological variability.

Association of isolated short femur in the mid-trimester fetus with perinatal outcome

OBJECTIVES

To evaluate the prevalence of fetal isolated short femur in a cohort of women screened for Down syndrome by the integrated test, and to compare the outcome of fetuses with isolated short femur in the mid-trimester with that of fetuses with normal femur length (controls).

METHODS

This was a retrospective cohort study of 1262 women booked for antenatal care and delivery at University College London Hospital. All women had integrated testing in the late first and early second trimesters and a detailed anomaly scan in the mid-trimester. All scan reports, screening results and neonatal data were analyzed statistically.

RESULTS

The fetal femur was short (< 5(th) percentile) in 5.1% of patients and 4.7% had isolated short femur. In pregnancies with isolated short femur, the birth weight was significantly lower and there were higher rates of small-for-gestational age (SGA) and low birth weight (LBW) infants, compared with controls (P < 0.01). The odds ratios for SGA and LBW in pregnancies with isolated short femur were 3.0 (95% CI, 1.5-5.9) and 2.60 (95% CI, 1.1-6.2), respectively. Isolated short femur was associated significantly with low levels of pregnancy-associated plasma protein-A (P = 0.001).

CONCLUSIONS

Isolated short femur in the mid-trimester fetus is associated with fetal growth restriction and SGA. In the context of normal Down syndrome screening and a normal anomaly scan, this marker should be regarded as a predictor for SGA, and fetal growth should be monitored during these pregnancies.

Prediction of birth weight by fetal crown-rump length and maternal serum levels of pregnancy-associated plasma protein-A in the first trimester

OBJECTIVE

To determine whether the first trimester crown-rump length (CRL), maternal serum levels of pregnancy-associated plasma protein A (PAPP-A) and free beta-human chorionic gonadotropin (fbeta-hCG) are independent predictors of birth weight.

METHODS

This was an observational study over 1.5 years in Chinese patients who underwent first-trimester combined screening for Down syndrome in a University fetal medicine unit. After excluding cases with multiple pregnancies, congenital malformations and in-utero deaths, the relationship between fetal CRL (expressed as standardized Z-score (Z-CRL)), maternal PAPP-A and fbeta-hCG levels (expressed as log(10) of multiples of the median) and birth weight (Z-BW) were analyzed by Pearson's correlation test followed by multiple regression to check for their independency. The predictive power of the independent predictors for small-for-gestational age (SGA, defined as birth weight < 10(th) centile) was then assessed using receiver-operating characteristics (ROC) curves, and the likelihood ratios were derived.

RESULTS

A total of 2760 cases were included. Z-CRL, log(10) PAPP-A(MoM), and log(10) fbeta-hCG were positively correlated with Z-BW (P < 0.0001), but only Z-CRL and log(10) PAPP-A(MoM) were independent predictors (P < 0.0001). The areas under the ROC curves of PAPP-A(MoM) and Z-CRL were 0.608 and 0.593, respectively (P < 0.0001). Likelihood ratios increased with decreasing PAPP-A(MoM) and Z-CRL, but were around 1 when the markers were at or above the mean.

CONCLUSION

First-trimester CRL and PAPP-A are independent factors that influence final birth weight. The lower the PAPP-A and the smaller the CRL, the higher the risk of a fetus becoming SGA. However, their predictive powers are not sufficiently good for them to be used alone for SGA screening.

First-trimester biochemical markers of aneuploidy and the prediction of small-for-gestational age fetuses

OBJECTIVES

To examine the clinical utility of the first-trimester biochemical markers of aneuploidy in their ability to predict subsequent delivery of a small-for-gestational age (SGA) infant.

METHODS

We examined singleton pregnancies with no chromosomal abnormality and with complete outcome data that had undergone screening for trisomy 21 by a combination of fetal nuchal translucency (NT) thickness and maternal serum free beta-human chorionic gonadotropin (beta-hCG) and pregnancy-associated plasma protein-A (PAPP-A) at 11 + 0 and 13 + 6 weeks' gestation. The biochemical markers were converted to multiples of the expected normal median (MoM) for a pregnancy of the same gestation. The association between free beta-hCG and PAPP-A and the incidence of SGA were assessed by comparing the relative incidence at MoM cut-offs and birth-weight centile cut-offs. At various marker levels the likelihood ratios (LR) for SGA were also calculated after excluding other adverse pregnancy complications.

RESULTS

There were 46,262 pregnancies resulting in live births with birth weight at or above the 10(th) centile, and 3,539 below the 10(th) centile for gestation (SGA). There was a significant inverse association between the risk for SGA and maternal serum PAPP-A MoM but not free beta-hCG MoM. At the 5(th) centile of the normal outcome group for PAPP-A (0.415 MoM) the odds ratios for SGA below the 10(th), 5(th) and 3(rd) centiles of normal were 2.70, 3.21 and 3.66 and the respective detection rates for SGA were 12.0%, 14.0% and 16.0%.

CONCLUSIONS

Low levels of maternal serum PAPP-A are associated, in the absence of an abnormal karyotype, with an increased risk for subsequent delivery of an SGA infant.

First-trimester ADAM12 and PAPP-A as markers for intrauterine fetal growth restriction through their roles in the insulin-like growth factor system

BACKGROUND

PAPP-A is a marker used as part of the most effective method of screening for chromosomal anomalies in the first trimester. ADAM12 is a recently discovered pregnancy associated member of the ADAM (a multidomain glycoprotein metalloprotease) family. Recently, ADAM12 has been shown as a potential marker for early screening for chromosomal anomalies. Both PAPP-A and ADAM12 have been identified as proteases to insulin-like growth factor binding proteins. In this role, they may have a regulatory function in controlling the amount of free bioactive insulin-like growth factor (IGF). We therefore wish to examine if the levels of either of these proteases are related to various growth related adverse pregnancy outcomes.

MATERIALS AND METHODS

PAPP-A and ADAM12 were measured in a subset of samples collected at 11 to 14 weeks as part of an OSCAR clinic screening for chromosomal anomalies. Follow-up of pregnancies screened between September 1999 and August 2003 identified 1705 pregnancies with an outcome of intrauterine fetal demise on or after 24 weeks, preterm delivery at 24-34 weeks or 35-36 weeks, very low birthweight (<1.5 kg), low birthweight (<2.5 kg), large birthweight (>4.5 kg), and birth weight below the 3rd or 5th or 10th centile for gestation. A series of 414 normal outcome pregnancies constituted the control group. Marker levels were adjusted for gestation and maternal weight and the log MoM of the markers were compared using t-test of unequal variance between the control group and the various adverse outcome groups.

RESULTS

ADAM12 and PAPP-A concentrations were reduced in low for gestational age birth weights and in all births with weights below 2.5 kg. There was a linear relationship between the severity of the IUGR and the decrease in PAPP-A and ADAM12. In the larger babies, only ADAM12 was found to be significantly increased in babies above the 90th centile of weight for gestation.

CONCLUSIONS

The results of our study are compatible with the proposed role of ADAM12 and PAPP-A in promoting growth and development by breaking down IGF binding proteins and causing the release of free IGF for uptake into cells to promote growth. In those cases that eventually result in poor fetal growth, levels of PAPP-A and ADAM12 at 11-14 weeks are significantly lower than normal-in this instance, lowered PAPP-A and ADAM12 would result in less free IGF being available for cell uptake and growth stimulation. Further studies may elucidate if screening using such modalities can lead to new potential treatments for poorly growing fetuses.

Fine mapping dissects pleiotropic growth quantitative trait locus into linked loci

A recurring issue in studies of quantitative trait loci (QTLs) is whether QTLs that appear to have pleiotropic effects are indeed caused by pleiotropy at single loci or by linked QTLs. Previous work identified a QTL that affected tail length in mice and the lengths of various bones, including the humerus, ulna, femur, tibia, and mandible. The effect of this QTL on tail length has since been found to be due to multiple linked QTLs and so its apparently pleiotropic effects may have been due to linked QTLs with distinct effects. In the present study we examined a line of mice segregating only for a 0.94-Mb chromosomal region known to contain a subset of the QTLs influencing tail length. We measured a number of skeletal dimensions, including the lengths of the skull, mandible, humerus, ulna, femur, tibia, calcaneus, metatarsus, and a tail bone. The QTL region was found to have effects on the size of the mandible and length of the tail bone, with little or no effect on the other traits. Using a randomization approach, we rejected the null hypothesis that the QTL affected all traits equally, thereby demonstrating that the pleiotropic effects reported earlier were due to linked loci with distinct effects. This result underlines the possibility that seemingly pleiotropic effects of QTLs may frequently be due to linked loci and that high-resolution mapping will often be required to distinguish between pleiotropy and linkage.