You may not know this, but if you go up to a pregnant woman and measure the distance from her pelvic bone to the top of the foetus, you will a/ know how far through her pregnancy she is; and b/ get a slap. If you do it enough times, and plead that it's for scientific purposes, then you are Dr John Baeyertz, an obstetrician and gynaecologist from Wanganui.
Dr B (as we'll call him, to lessen the chances of misspelling his surname) realised that the traditional methods of calculating a baby's due date - either by calculating from the date of the last menstrual period, or by the date of conception - were both error prone and unreliable. He had of course heard of the technique of using the measurement from the symphysis to the fundus (doctor-speak for 'pubes to the top of the baby'), but likewise there were problems with this technique too.
Dr B realised that with the advent of AI – artificial insemination – he could accurately measure the time between conception and delivery, and formulate a precise technique to be used for all women. He spent over 13 years collating the data from 127 pregnancies, including some twins, some abnormal pregnancies, some women who were early, some who were late… from his studies he gained a wide cross section of pregnancy in New Zealand, or at least, in Wanganui.