A formula giving the instantaneous birth rate is derived for egg bearing species or species which bear their young. It is shown that the instantaneous birth rate can be calculated when the egg ratio or pregnancy rate and the development time are known. The new method is compared with earlier ones by Edmondson and Caswell: the latter applies incorrectly an earlier formula by Leslie while Edmondson’s formula gives biased results, the bias depending on the length of the development time.