Hi, I am an American Board Certified Neurologist with vast experience in diagnosing & treating all types of neurological & other medical/OB Gyn conditions including the concerns of “duration of pregnancy” you have mentioned here and I am very glad to help you with your health questions.
Please see that the first heart beats are identified on an average around 6 weeks of pregnancy.
But there is a normal range, not all fetus heart beats are identified for the first time at 6 weeks, in some it may be identified bit earlier and in some bit later too
So fetal heart beat cannot be used to accurately measure the duration of pregnancy, rather it will give only an approximate idea about the duration of the pregnancy
When first heart beats are identified in a fetus we can only say the pregnancy is around 6 weeks duration but we cannot say with certainty it is indeed 6 weeks because there is a normal range as mentioned above (for example in some the heart beats may be identified as early as around 5 weeks too)
For measuring the duration of pregnancy the better indicators are CRL (Crown Rump length) seen on ultrasound, and this is the most useful parameter to estimate the duration of pregnancy in the first about 13 weeks of pregnancy and after this period other ultrasound measurements like head circumference, biparietal diameter, femur length measurements etc become more useful measurements for assessing fetal growth and estimating the duration of pregnacy.
Once again to your main question, please see the time when heart beat is identified for the first time is around 6 weeks so based on this we can say the pregnancy is approximately about 6 weeks in duration but this is not the most reliable parameter to estimate the duration of pregnancy for the reasons mentioned above, and if you want to more accurately estimate the duration of pregnancy then one of the above mentioned ultrasound findings will be more helpful than the time when the fetal heart beats are felt for the first time.
Please let me know if you have any further questions, and please click a positive rating (preferably 5 Star/Excellent) to credit for the time & effort put by me in answering your health question, and this is the only way I will be credited for my service to you. If you have follow-up or additional questions you can ask them after the rating too.