Does the UIN pup column uniquely identify each individual?
Each pup has a unique identity number that stay stays with it throughout its life (links to a Dnnn number when homed), the colours refer to nail varnish that is used to identify each puppy visually for weighing, health etc. otherwise it could be very difficult to tell the pups apart.
The UIN Pup column is set up as a drop down list that shows the Id Mark colour but returns the UIN. The Id Mark column just displays the nail varnish colour so that it is easy to see who has and who has not been weighed.
Assumptions on which I am basing this answer:
a) this data is collected at most once a day for a given pup
b) Weighed Date is a field of type date.
Ideally, I would set up this relationship:
Fields such as ToB, UIN Pup, Sex and ID Mark would be defined in Pups. The other fields in your screen shot would be defined in WeightStatistics. I'd use an internally generated ID field defined in Pups to link a record in pups to all the weight records for that pup in WeightStatistics.
But whether you do or or have done that or not, the following setup can allow you to access a pup's previous measurement data:
WeightStatistics::UIN PUP = WeightStatistics|SamePup::UIN Pup AND
WeightStatistics::Weighed Date > WeightStatistics|SamePup::Weighed Date
Weightstatistics is the name I am giving a second occurrence of your table where you record these weight measurements. Double click the relationship line between these two occurrences and specify a sort order for WeightStatistics|SamePup::Weighed Date that sorts on this field in descending order. This makes the record for the same pup with the latest weighed date that is less than the current record in WeightStatistics the "first" related record and this will be the immediately previous record for this pup.
Now you can simply refer to fields of WeightStatistics|SamePup to access data from that previous record.