However, the National Academy of Clinical Biochemistry (Baloch et al. 2003), as well as authors cited by Goodman et al. (Krassas et al. 2010), recommend using trimester-specific reference ranges because thyroid hormone levels change during the course of pregnancy.
We used trimester as a covariate in models for pregnant and first-time pregnant females and defined it as follows: first trimester, < 4 months; second trimester, 4 6 months; and third trimester, > 6 months.
We used trimester divisions of 1 13 weeks, 14 26 weeks, and 27 weeks to birth; similar definitions have been applied elsewhere (e.g., Parker et al. 2005).
Most studies on IUGR used trimester-specific exposure windows.
This is, of course, a consideration in all studies that use trimester-specific or 90-day averaged temporally but not spatially resolved pollution, because time periods shorter than 365 days overlap with season.
We also evaluated timing of exposure by trimester of pregnancy using a trimester × cold season interaction term, but the small sample size restricted our ability to detect an interaction effect.
We repeated this analysis using each trimester as the initial reference trimester (i.e., trimester a).
First and second trimester induced abortion were compared using first trimester abortions as the reference group.
If the environmental pollutant considered can restrict fetal growth as early as the first trimester, correcting gestational age using first-trimester ultrasound measurements will erroneously shorten the gestational age of these small-for-gestational-age fetuses.
To derive a prediction rule for preeclampsia and early onset preeclampsia requiring delivery <34 weeks using first trimester maternal, ultrasound, and serum markers.
Analyses performed using first trimester serum samples have resulted in negative findings [ 19, 21, 22].
