PIP: The relationship between birth order and the risk of perinatal mortality has been explored in some detail. Different populations show somewhat different relationships, but in Europe and North America the predominant pattern is one of perinatal mortality falling from the 1st to the 2nd or 3rd birth and rising in subsequent births. Several authors have cautioned against interpreting the J shaped curve as showing that a high order birth is inherently more risky than a low order birth. An alternative explanation may simply be that the group of women presenting for, e.g., a 5th birth, contains a relatively high proportion of high risk women, who were at higher than average risk even in their 1st pregnancy. It is possible that the more successful reproducers withdraw from childbearing earlier, leaving, at each successive birth order, a greater concentration of poor reproducers. Some researchers have defined cohorts of women and compared the mortality rate for each cohort at different birth orders. However, this method produces its own distortions as long as the propensity to continue to another pregnancy is related to the outcome of previous pregnancies. A new approach is suggested for assessing the relationship between birth order and pregnancy risk. The term "birth order" is replaced by the term "parity" where parity is defined as the number of previous live births, and stillbirths of 28 weeks or more. An experimental model measuring perinatal mortality rate (PMR) was set up for an animal population. By contrast with this experimental model, the separate PMRs relating to each human reproductive history have been averaged together using wrong weights. If it were possible to estimate the correct weights one might yet be able to achieve a true estimate of parity risk in a human population. It is necessary to observe a cohort of women from their 1st pregnancy through to the end of their reproductive life--a period of about 25 years. Measuring parity risk for a real life population will enable assessment of the risk of a further delivery for an individual woman of given parity. Data on the reproductive histories of 16,583 West Jerusalem women were made available. Analysis for multiparae confirms that reproductive history is the most important factor in predicting risk in the current pregnancy. Of the elements of reproductive history examined, the incidence of 1 or more previous poor pregnancy outcomes showed the strongest association with increased risk in the current pregnancy. The risk of a poor outcome at the 1st pregnancy appears to be twice that at subsequent pregnancies, but no change in risk is observed between the 2nd and 5th birth orders. If there has been a previous poor pregnancy outcome, the risk of another is increased 4 fold. The risk of a poor pregnancy outcome at the 1st birth is substantially higher than the risk of a 1st poor outcome at higher parities.