She will probably be due in 1-2 week of April. But, if you did not do a progesterone test for the exact date of ovulation, then you will need to measure temperature. Generally twice per day at the same times, keep a chart.
What I find is that they drop temperature gradually, from 101.6 say to 99.3 or so, and then within 12 hours, they will dip usually down around 98 degrees (I am measuring with a cheap digital thermometer so accuracy may not be the greatest but what you are looking for is the rate of change.) Usually if they drop a degree, down below 99 in 12 hours, then they will whelp within 24 hours.