370 SPO Abstracts

340 THE DEPENDENCE OF THE PLACENTAL/FETAL RATIO ON FETAL WEIGHT. R.B. Kurzel. M.D .. U.C.L.A.lL.A.C. - Olive View Medical Center, Sylmar, CA

341

Because of the enormous range in placental weights (P­Wt) vs gestation, studies of the placental/fetal weight ratio (PIF) are felt to be more meaningful than P-Wts in predicting the physiologic adequacy of the placenta. Fetal (F-Wt) and P-Wts were obtained from 1633 singleton liveborn pregnancies prepared according to Thomson et al. Mean P-Wt and P/F were determined for 500 gm F­Wt intervals, from 500-6,000 gms, using a fixed F-Wt as the criteria to study the variation in these parameters. P/F's were obtained from the mean F-Wt and P-Wt for each weight interval. RESULTS & CONCLUSIONS: (1) For the entire population, mean F-Wt = 3394 gm, mean poW! =640 gm, and P/F =0.189 = 115.3 (2) Mean P-Wts increase with increasing F-Wts. (3) For a given F-Wt group, the P-Wt follows a Gaussian distribution, with a wide range, '" 900 gms. (4) At a given F-Wt, fetal sex does not influence P-Wt. (5) The P/F ratio vs F-Wt shows a decreasing hyperbolic trend, achieving the assymptote (P/F =0.190) for F-Wts ~ 2500 gms (term). The leveling off of P/F with F-Wt indicates efficiency achieved at term (minimum in P/F), and supports the concept of Placental reserve.

Metabolism of Cocaine by N-demethylase in Rat Placentae: An Induced Placental Enzyme System. Bertis B. Little, Ph.D.: Daniel A. Roe, B.S.: R. William Stettler, M.D:, Van R. Bohman, M.D: Dept. of Ob/Gyn, The Univ. of Texas Southwestern Medical Center, Dallas, Texas.

Previous investigators reported cholinesterase activity (CA) of placenta in vitro. Four treatment groups were incubated with cocaine (C) over 4 time periods: placental microsomes (PM) + C, PM + DFP (anticholinesterase) + C, PM + C + butyryl-cholinesterase (BC), and a blank (C only). Gas chro­matography was used to quantify C (limit of quantitation-LOQ = 19 ng/ml) and metabolites. BC enhanced C metabolism to ecgonine methyl ester (EM E). More than 40% of C was metabolized to NC by rat placenta when DFP suppressed CA (FIG.).

-0----0-

(::C:.".OI" NC:C.JtII.OFP

NC is produced by hepatiC N-demethylase action on methyl­bearing nitrogen in C, suggesting that placenta as well as liver has this capacity. Hence, this biotransformation of C may be a primary metabolic pathway induced in the cholinesterase deficient placenta. This has clinical implications because NC is 9-fold more active physiologically than C or EME.

January 1992 Am J Obstet Gynecol

342 IS PLACENTAL GROWTH OPTIMAL?

343

LJ Qroome, JM Benanti. University of Arkansas for MedIcal SCIences, Little Rock, Arkansas.

At the present time the relationship between fetal well-being and placental growth is largely unknown, although abnormally small or large placentas are thought to be a marker of fetal compromise. Oxygen (0 ) delivery to the fetus is reduced, because of a reduction in transport area, if the placenta is too small; in addition, the hillh rate of 02 consumption by a large placenta can SImilarly limIt fetal 02 delivery. This relationship between placental size and 02 exchange raises the question "Is there an optimal placental growth pattern which maximizes fetal oxygenation?" Ordinary non-linear differential equations were derived to describe the axial P02 profiles in the maternal (M) and fetal (F) streams for concurrent flow. A Fibonacci search routine was used to assure consistency between umbilical artery and venous P02 profiles and fetal 0.2, consumption (Vr). Model parameters were well within the range cited for a 3 kg sheep fetus: 1000 ml/min (M) and 500 ml/min (F) blood flow; 18% (M) and 8% (F) flow shuntsj. 2 ml 02/min-mmHg placental 02 dlffusivity; data 01 Edelstone (AJOG, 1985) for V f ; and Hill equations (AJP, 1972) for the oxyhemoglobin binding curves. Based on this mathematical model, we found that 02 delivery to the sheep fetus is maximized at a placental mass (mp) of 800 gms. Furthermore, (6 POj)UmV IA m IS an order of magnitude greater for placentas wefghing <800 mg than for placentas weighing >800 mg (0.04 vs 0.003 mmHll/gm), suggestinll that a smaller placenta limIts fetal oxygenatIOn to a greater extent. Conclusion: Fetal oxygenation may be an important factor regulating placental growth.

THE POST-TERM PLACENTA: SMALL OR NOn MP Dombrowski. HM Wolfe, AA Saleh, RJ Sokol, Depts of Ob/Gyn and Pediatrics, Wayne State Univ./Hutzel Hosp., Detroit, MI

Placental weight has been directly related to total placentofetal metabolism. Placental growth has been reported to plateau beyond 40 menstrual weeks, with the small, post­term placenta associated with "placental insufficiency". The purpose of this study was to examine placental growth beyond 40 weeks using obstetric estimates of gestational age IGA-OB) based on LMPs, but corrected by fetal ultra sounds and confirmed by Ballard exam, and then compare data based solely on LMP (GA-LMP). Placental weights were obtained from a perinatal database of 33,135 viable, singleton, structurally normal neonates.

GA-OB GA-LMP weeks n weight ± S.D. n weight ± S.D.

40 9,648 678 ± 144 4,689 671 ± 146 41 3,538 700±146 3,782 686±153 421,353 715±152 2,442 687±154 43 130 781±198 1,287 680±155 44 3853±103 815 684±150 45 0 578 690±142 46 0 306 691 ±157 >46 0 356 679±155

Consistent with previous reports based on GA-LMP, there was a plateau beyond 40 weeks; weights at 43 weeks were similar to 40 weeks (p > .08). However, when dated by GA-OB, placental weights increased from 40 to 44 weeks (p < .0001). The incidence of pregnancies £ 42 weeks was significantly less (p < .0001) when gestational age was corrected by ultrasound. We conclude 1) typical placental growth continues beyond 40 weeks 2) gestational dating solely by LMP markedly overestimates the true incidence of post-datism and distorts apparent placental growth pattern 3) the concept of placental insufficiency requires re-examination.