To the Editor:
Cigarette smoking influences the concentrations of certain markers in maternal serum during pregnancy. An approximate 20% reduction in pregnancy-associated plasma protein A has been noted in first-trimester maternal sera of smokers compared with nonsmokers. Furthermore, inhibin A is increased 40%–60% in maternal serum of second-trimester smokers. Adjustment for smoking with these 2 markers is important to reduce the false-positive rate in estimating the individual risk for Down syndrome.
Pregnancy-specific β-1-glycoprotein (SP1)1 was first described 3 decades ago as pregnancy-associated plasma protein C (1) and Schwangerschafts protein 1(2). SP1 is synthesized by the syncytiotrophoblast and secreted into the maternal circulation, where the concentration increases continuously during pregnancy. The SP1 concentration in maternal serum is a marker of Down syndrome, with a reduced concentration noted early in the first trimester and an increased concentration in the second trimester (3)(4). Furthermore, serum SP1 is decreased in pregnancies with fetal growth restriction. Smoking has been reported to reduce the SP1 concentration in late pregnancy (>30 weeks), especially in pregnancies with fetal growth restriction. We examined the effect of smoking on the SP1 concentration in first-trimester maternal serum.
We randomly selected 500 cases from a cohort of 4092 Caucasian, live-born, singleton pregnancies that were consecutively enrolled in the first-trimester combined screening program for Down syndrome between May 1, 2005, and May 31, 2007, at Holbæk Hospital (primary hospital), Denmark. We used the Gestation Related Optimal Weight algorithm to restrict selection to women giving birth after 37 completed weeks and with a birth weight between the 10th and 90th percentile (5). Maternal demographic data (including smoking status) were obtained from the local fetal database (Astraia Software). The median age was 29.0 years (range, 17–43 years). Blood samples were collected in 5-mL non-anticoagulated tubes and centrifuged; the serum was removed and stored at 4 °C until further analysis. Blood samples were drawn at a gestational age of 8 weeks to 13 weeks plus 6 days (median, 10 weeks plus 1.5 days). The use of samples and clinical information for evaluating screening performance and parameter distributions was part of a QC program and was carried out in accordance with the guidelines of the Danish Prenatal Registry at Statens Serum Institut, Copenhagen, Denmark.
A previously described (4) in-house time-resolved immunofluorescence assay was used to measure SP1. Calibrators were referenced against WHO International Reference Preparation 78/160. All samples were analyzed in duplicate and reanalyzed if the CV exceeded 10%. The interassay CV was 8.6%, and the maximum intraassay CV was 4.4%. SP1 concentrations were log10-transformed to approximate a gaussian distribution, and multiple regression analysis, including gestational age and maternal weight, was performed to establish a normal median for SP1. The normal medians were then used to transform SP1 concentrations into multiples of the normal median (MoMs).
The 500 cases included 421 nonsmokers (84.2%) and 79 smokers (15.8%). A small difference (P = 0.041) in the median age (29 years vs 28 years) was noted between the nonsmokers and the smokers, respectively. No significant difference was noted in the median maternal weight (71 kg vs 69 kg) or the median gestational age for blood sampling (73 days vs 72 days). Table 1⇓ . shows the SP1 MoM values according to smoking status. The SP1 MoM median was reduced by 27% in smokers, compared with nonsmokers (P < 0.0005). The impact of smoking might have been underestimated because the validity of self-reported smoking status is questionable in the nonsmoking group; however, the percentage of smokers in the 500 cases (15.8%) is in accordance with national statistics, which indicate 15.9% smokers during pregnancy in 2005.
This report is the first of smoking influencing the SP1 concentration in first-trimester serum. It is likely that smoking has confounded previous results in studies of first-trimester SP1. In estimating individual risk for Down syndrome and growth restriction based on the SP1 concentration in first-trimester serum, adjustment for smoking might be important to reduce the false-positive rate and to improve screening performance.
Author Contributions: All authors confirmed they have contributed to the intellectual content of this paper and have met the following 3 requirements: (a) significant contributions to the conception and design, acquisition of data, or analysis and interpretation of data; (b) drafting or revising the article for intellectual content; and (c) final approval of the published article.
Authors’ Disclosures of Potential Conflicts of Interest: No authors declared any potential conflicts of interest.
Role of Sponsor: The funding organizations played no role in the design of study, choice of enrolled patients, review and interpretation of data, or preparation or approval of manuscript.
- © 2010 The American Association for Clinical Chemistry