These plastic pregnancy wheels are durable and precise. Based on the last menstrual period, they calculate a pregnant woman's due date. They also include guides for determining date of conception, ovulation, suggested medical tests, ultrasounds, and more! They include baby's estimated weight in grams and baby's crown-to-rump length in millimeters. These pregnancy wheels were previously available to only medical personnel, but now anyone can order this valuable pregnancy calculator. Perfect for predicting ovulation too for women who are trying to conceive. Made in the U.S.A.