Can a baby's due date be a month off when determined by an ultrasound?

A lot of women have questions about their due date and how the doctor determined it. And it's usually done 1 of 2 ways. First of all, it's important to seek early prenatal care before 13 weeks gestation. And at that point, the doctor will ask you if you've had regular cycles in the past and when your last period happened. If you know what date that was and you've been regular in the past, then the doctor can pretty reliably calculate a due date off of that, because a woman is fertile for 6 days out of each cycle - 5 days leading up to ovulation and on the day of ovulation. And we know that because you had a period, you weren't pregnant at that point in time, so you got pregnant during that next cycle when you ovulated and egg and sperm met up. If you had irregular cycles in the past or you're not sure when your last period was, then the doctor may send you for an early ultrasound. And this is where they measure the size of the growing fetus, and look at growing structures, and determine your gestational age from that, because a fetus will grow at a very consistent rate during the 1st trimester. So if your baby is measuring 8 weeks, you're reliably 8 weeks along.

Now there's also some confusion surrounding the amount of time the baby has been growing versus your gestational age. The entire gestational period is 40 weeks. However, a baby only grows for 38 out of those weeks, because the first 2 weeks of the gestational period are from the time of your first period till the time of ovulation and conception. So the baby is not actually growing for those first 2 weeks, so that confuses some people. They get caught up in the fact that the doctor told them they were 8 weeks, but the baby has only been growing for 6 - you're just 8 weeks gestation. We standardize it across the board.

Now another commonly asked question is, "Will my due date change if my baby is measuring abnormally small or large later on in pregnancy?". Now I told you that in the 1st trimester, a fetus will grow at a very consistent rate, but thereafter, it can differ. And there are underlying causes that can make a baby be abnormally small or abnormally large, and those are the things that a doctor needs to consider - not changing the due date. So let's say, for example, that you have an ultrasound at 8 weeks, and the doctor knows reliably how far along you are, and what your due date is. Later on in the pregnancy, if you have an ultrasound at 28 weeks for whatever reason and the baby is measuring 32 weeks, they won't just change your due date. And there's a couple of reasons for that.

Number 1, a doctor needs to consider underlying causes and decide if further investigation or treatment is necessary for that. And the 2nd is that if the doctor actually changed your due date and put you a month ahead, that's not going to change how much time the baby has spent growing in the womb, and their organs won't be fully mature when the baby is full-term. If you have more specific questions about your situation, don't hesitate to talk with your OB provider, and they'll be able to give you the best advice and information based on their knowledge of your situation. And if you have more questions for me in the future, feel free to ask them on our Facebook page at http://www.facebook.com/IntermountainMoms, and recommend us to your friends and family too.

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