The doctors and midwives use a standard set of three methods to count pregnancy weeks – based on LMP, using ultrasound, and through physical examination. Though the actual timing of delivery cannot be known, the three-step process helps you predict the likely date of birth.
You can know your pregnancy week by the size of the uterus. Around 12 weeks, your medical practitioner can feel the uterus (fundus) above the pelvis. After 18 weeks, the distance (in cm) between the fundus and pubic bone will be the same as the number of weeks from the last menstrual period. In about 20 weeks, the fundus is at the umbilicus. This gives a rough idea about the pregnancy week. However, this is not always the right way to predict the gestational age due to factors such as twin pregnancy, obesity, uterine tumors, or the fundus being higher or lower
Ultrasound works even when you are not aware of the LMP or if your periods are irregular. It is based on a series of measurements of the gestational sac and the fetus.
The sonographer measures the baby from head to toe. This gives the crown rump length or CRL. It helps the doctor estimate the pregnancy week based on the growth of your baby