11. Ageing of foetus
There are two major reasons why the age of a foetus needs to be determined:
- In order to determine whether a ewe is pregnant as a result of an artificial insemination or following the implantation of an embryo or pregnant to a "backup mating"
- In order to separate earlier from later lambing ewes, an important procedure for single-bearing and single-lambing ewes.
Accurate identification of ewes becoming pregnant earlier or later in a natural joining is complicated by the influence of gestation length. This complication is best explained in the following way;
All ewes that join on one day will in fact lamb over 7 to 10 days. This is a natural phenomenon which occurs in every flock and is due to normal variations in gestation length between ewes.
Where a flock of pregnant ewes is scanned to differentiate the earlier from the later lambing ewes, some of the ewes scanned as the later lambing ewes will lamb before the last of the earlier lambing ewes has lambed, and vice versa. This "overlap" is unavoidable, and accurate ageing of the pregnancies is required to keep "overlap" to a minimum.
The requirements to enable the highest level of scanning accuracy have been detailed elsewhere.