When it comes to calculating pregnancy weeks, healthcare professionals typically use the first day of your last menstrual period (LMP) as a starting point. This is because it can be difficult to determine the exact date of conception. Based on your LMP, they estimate that ovulation occurred around two weeks later, which is when fertilization likely took place.
However, it's important to note that every woman's cycle is unique, and not all women ovulate exactly two weeks after their period starts. Additionally, sperm can survive in the body for up to five days, so conception may occur several days after intercourse.
In your case, since you had a negative blood test on May 22 and a positive home pregnancy test on May 3, it's possible that you conceived sometime between those dates. The nurse likely estimated your gestational age based on your LMP and assumed ovulation occurred around two weeks later.
It's worth mentioning that early ultrasounds are often used to confirm gestational age more accurately. These ultrasounds measure the size of the embryo or fetus and can provide a more precise estimate of how far along you are in your pregnancy.
If you have any concerns or doubts about the accuracy of your estimated gestational age, it's always best to consult with your healthcare provider. They can provide further guidance and perform additional tests or ultrasounds if necessary.