Hi I just recently found out I'm pregnant with twins. When I went to the doctor I didn't know my exact period so he measured the babies for the crl. Baby A was 1.72 (top sac) and Baby B 1.79 (bottom sac. Both crl dated back to 7weeks 2days. Although I was very excited it was no way I couldve got pregnant then because the last time I had sex was the week of the 5 - 12 of Aug & that meant more than 3 weeks ago. Im very confused. My period usually comes on the 13th but it was late didn't come until the 20 or 21 I'm certain.My period was normal just late so I thought. The crl measured back to this date but that isn't what the computer calculated which was 7.2weeks. Im not understanding why this so off. Did I get pregnant right before my period? And the sperm survived that long? Can someone help please . Thanks in advance .

Calculating conception dates is an inexact science. Its just educated guesswork. Discuss these things with your doctor.

These are dates based upon several metrics of the body, head size, body length, and length of various other bits of the fetus. As Scott said it isn't an exact science.Being twins this could be off too.

It also depends on how they measure the age, whether it was from the beginning of the cycle you got pregnant or approx conception/implantation date. These are fluid numbers and have no more relevance then to give them an idea of how to evaluate the condition of your babies.

In the end all it can tell you is the approximate time you will give birth, plus or minus a few weeks. The rest is for the doctors to make sure they're fine. Pay the numbers no mind.

Also keep in mind that when they calculate dates, they calculate from the start of your last period. Gestation is 40 weeks, from the last period, until you give birth. For 2 of those weeks you're not actually pregnant. Is this making any sense? Also, unless you know exactly when you ovulated by taking basal temperature for at least 6 months every single day, there's no way to know exactly when in your cycle you conceived. It is possible to get pregnant while on your period, and any time after.