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The Center for Fertility and Gynecology

Congratulations on your pregnancy!....When are you due?

By on August 23, 2013

“What is your due date?” is the most common question people ask once they find out that someone is pregnant.  It seems to be human instinct to want to know when the baby is coming.  While, it is common knowledge that pregnancy lasts “9 months”, determining the precise gestational age and an accurate date of delivery is more complicated than most people think.  This blog entry will explore how gestational age and due date are determined and how that determination differs in pregnancy conceived through in vitro fertilization vs. other means.

The average human gestation lasts 38 weeks from the time of fertilization until the day of birth.  In the medical literature, this is referred to as the embryonic age.  For those undergoing in vitro fertilization (IVF), this age can be determined with great accuracy because the time of fertilization is narrowed to an 18-24 hour period.

Once an oocyte is removed during egg retrieval, it either undergoes ICSI within a short period of time, or is incubated with sperm.  The next day, 18-24 hours later, the egg is checked to see if fertilization has occurred (it develops 2 pro-nuclei).  In this way, the moment of fertilization is narrowed to a small time frame.

For patients undergoing intrauterine insemination (IUI), determining embryonic age is less precise than IVF, but more precise than spontaneous pregnancy.  Usually an IUI is performed following a positive ovulation prediction kit, or approximately 36 hours after taking an HCG “trigger” shot.  In both these scenarios, the time of ovulation can be narrowed to less than 24 hours.  The time of fertilization however, is harder to determine.  It is assumed that fertilization, which takes place in the Fallopian tube, occurs within 24 hours of  ovulation, but because this all happens within the pelvis, we have no way of knowing for certain.

Patients who become pregnant without any monitoring (ultrasound, ovulation prediction kit, basal body temperature, etc.) generally use menstruation to determine due date and age of the pregnancy.  The average human pregnancy lasts 40 weeks from the first day of the last menstrual period to the day of delivery.  This assumes that ovulation occurs 14 days after the first day of the period.  Accordingly, this age, which is called gestational age, is 2 weeks greater than the embryonic age. 

Regardless of the method of becoming pregnant, the gestational or embryonic age must be confirmed by early ultrasound.  In the first trimester, the length of the embryo closely correlates with the age.  This correlation is so strong that even if a person is sure of their last menstrual period, gestational age will be changed if necessitated by ultrasound measurement.   

If you have questions about pregnancy dating or human reproduction in general,  please don’t hesitate to contact us, we’d be happy to speak with you.

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