What is considered 1st base dating
Ultrasound Dating Ultrasound can be used to date pregnancies, especially when the LMP is not known (for example, pregnancy after a delivery but before a menses occurred or irregular menses without predictable ovulation).
Ultrasound dating is based on a series of measurements of the gestational sac in very early pregnancies and of the embryo or fetus itself as pregnancy progresses.
The average time between menstrual periods is about 28 days, and ovulation occurs about 14 days before the next period begins.
So when we say a woman’s estimated gestational age is six weeks’ LMP, we mean she conceived about four weeks ago, around the time of ovulation.
One reason for not dating pregnancies from the day of conception is that we cannot know that day exactly (excluding cases of assisted reproduction), but we can know the first day of LMP, based on what a patient reports to us.
However, the timing of the actual delivery is not so predictable.
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Health-care providers define the stage or length of pregnancy differently than many people might think. Logically, one might imagine we represent a pregnancy by how much time has elapsed since conception.
Instead, we most commonly date pregnancies from the first day of the last menstrual period (LMP).
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Some women will deliver on their due date, others before, and others after.