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  • Q&A with Australian Health Practitioners

    How do I figure out the date of conception?

    Related Topics
    Pregnancy & Breastfeeding Trying To Conceive Women's Health
    I hear different theories from everyone.
    My baby is due on the 24th of September 2013.
    What is the DATE of conception?
    • 2 answers
    • Dr Karen Osborne
    • Dr Gary Sykes
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    Dr Karen Osborne

    GP (General Practitioner)

    Sydney GP specialising in Reproductive & Sexual Health for both women and men. At Clinic 66, we deal with a wide range of reproductive and … View Profile

    Your date of conception will be approximately 2 weeks after the first day of your Last Menstrual Period (LMP) assuming you have a regular 28 day cycle. So, in your case, if you are due on 24th Sep, you would've conceived around about New Year's Day. This is assuming that your dates have not been changed based on scan results. Of course, every woman is unique and has a slightly different cycle so this is just an estimate. The reason that drs calculate pregnancy dates from the LMP not the date of conception is because, unless you have had IVF, it is very difficult to be sure.
    Congratulations on your pregnancy!

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    Dr Gary Sykes

    Obstetrician

    Dr Gary Sykes is a graduate Bachelor of Medicine and Bachelor of Surgery (M.B, B.S.) from Sydney University. He commenced his initial postgraduate training at … View Profile

    That is not an easy question to answer.

    The reason why we use the first date of the last menstrual period (LMP) in calculating expected date of confinement (EDC) is because most women know their LMP date. But most women don’t know the actual date of ovulation. The exception is women undergoing subfertility management, when usually the date of ovulation / egg retrieval / egg transfer is known.

    Obstetricians work on the assumption that ovulation has occurred 2 weeks after the LMP. So when we calculate the EDC (40 weeks after day 1 of LMP) the baby is actually 38 weeks old (assuming conception occurred 2 weeks after LMP).

    Ultrasound scans are very helpful in checking all this. An ultrasound scan about 10 weeks pregnant is usually most accurate. Earlier scans can have error because the baby is too small to accurately measure. I have established baby viability (by observing the baby’s heart beating regularly) when the ultrasound length of the baby measured 0.2cm which meant the baby was 5 weeks and 3 days old based on LMP, or was actually 3 weeks and 3 days old)

    So it is not unusual to move the EDC based on scan measurements. Moving an EDC implies it si considered ovulation was not 2 weeks after LMP but say 3 weeks or whatever is established to be the case by scan. It a woman has an irregular and longer menstrual cycle then this supports this, but it may also be true for the woman with a 28 days cycle.

    Often a woman knows when she is ovulating because she detects a change in her mucous (from gluggy to stringy – called the spinnbarkheit effect), often a twinge of ovary (ovulation) pain and occasionally a spot of blood. Knowledge of sex timing is not that helpful in establishing conception as sperm can live for days.

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