Abstract

Time to pregnancy is the duration from a couple starts trying to become pregnant until they succeed and is considered one of the most direct methods to measure natural fecundity in humans. Statistical tools for designing and analysing time to pregnancy studies belong to the general area of survival analysis, but several special features require special attention. We focus here on starting from a cross-sectional sample of couples currently trying to become pregnant, using current duration as basis for estimation. This corresponds to using the backward recurrence time as basis for the inference, and here the preferable statistical model turns out to be the accelerated failure time model.

The inference is quite sensitive to observations near zero; indeed Woodroofe and Sun pointed out that the nonparametric maximum likelihood estimator is inconsistent at zero. Switching to parametric models, we are faced with balancing between stability or flexibility. The former represented by a simple mixed exponential model such as the Pareto distribution, the latter by a much larger class of distributions suggested by Yamaguchi in a sociological context.

We are collaborating with a group of epidemiologists, statisticians and demographers in France where we have helped design and analyze a large telephone survey among French women based on the current duration approach. The talk will briefly mention several practical features that one has to take into account but we will focus on the statistical problems and present various simulation studies highlighting the sensitivity problems.

References:

Keiding, N., Kvist, K., Hartvig, H., Tvede, M. & Juul, S. (2002). Estimating time to pregnancy from current durations in a cross-sectional sample. Biostatistics 3, 565-578.

Scheike, T. & Keiding, N. (2006). Design and analysis of time to pregnancy. Stat. Meth. Med. Res. 15, 127-140.

Slama, R., Ducot, B., Carstensen, L., Lorente, C., de La Rochebrochard, E., Leridon, H., Keiding, N. & Bouyer, J.(2006). Feasibility of the current duration approach to study human fecundity. Epidemiology 17, 440-449.