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Due date based on conception date

Calculating your due date

No matter how much planning goes into conceiving a baby, it is important to remember that pregnancy is not an exact science. The closest we can get is when a woman has undergone fertility assistance, and then it can be possible to pinpoint exactly when conception has occurred. From this a more accurate estimation of due date based on conception date is possible.

In normal conception however, knowing the exact moment when the sperm fuses with the egg is impossible.

How is the due date calculated?

When conception has occurred naturally, estimating the baby’s due date is generally done in the same way the world over. Using a pregnancy calculator provides a good estimate of the baby’s due date based on a mother’s menstrual history, rather than when conception has occurred. The age of the baby or its gestational age is estimated by calculating the first day of a mother’s last normal menstrual period and then adding 40 weeks to this date. This is dependent on a mother having a normal 28 day cycle, when it is assumed that conception has occurred around 14 days after the start of her period.

But many women have menstrual cycles which are a few days longer or shorter than this. This is why it is important to take individual factors into account when estimating the baby’s due date.

If a woman has had fertility assistance it can be easier to predict the baby’s due date. This is because with IVF it is clear when fertilisation occurred, the rate of the embryo’s development and exactly when it was implanted into the uterus. Blood tests which measure a mother’s varying hormonal levels are another way to estimate the stage of her pregnancy.

Who’s counting anyway?

Typically, a human pregnancy lasts for somewhere between 38 and 42 weeks. This is why 40 weeks or 280 days is seen as “the norm”. Interestingly, though we all think of pregnancy as lasting for 9 months, it really is closer to 10 calendar months. This is because, for all sorts of reasons, we start counting the weeks of pregnancy from the time of the last period, not two weeks later when conception is most likely to have occurred.

It is also usual practice to refer to the due dates as the Estimated Date of Confinement (EDC), or the Estimated Date of Delivery (EDD) – the word estimated is an important one to remember. Every mother and her baby are individuals and development towards maturity is dependent on lots of factors, not just calendar days.

Another way of estimating a baby’s due date is to count back three months from the first day of the last normal period and then add seven days to this. For example, if a mother’s period started on December 1st, then three months back is September 1st + 7 days = due date of September 8th. This is another way of estimating the 40 week gestation period.

But what if I know when I conceived?

Sometimes it is possible to estimate the due date based on (likely) conception date if intercourse has been infrequent. In some relationships, a couple’s physical proximity, let alone their intercourse habits are sporadic and irregular. Military and mining families, couples who work away from home and away from each other, are, more often than not, all familiar with the logistics of sustaining an intimate lifestyle.

This is why being able to pinpoint exactly when conception occurred with more certainty can be easier for some couples. But again, date based calculations are not exact and at best, should be seen as a “guesstimate”, rather than being absolutely precise.

Due Date Calculator

Fill in your details below to calculate your due date:

* Please remember this result is only an estimate. Your baby will arrive when he/she is ready.
Do consult your healthcare provider to determine your correct due date.

But I’m sure I know when I ovulated!

Whilst it is true that many women are so acutely tuned into their bodies that they know with almost absolute certainty when they have ovulated, being so sure about when they conceived is less exact. Ovulation and conception don’t occur at the same time; neither does conception happen immediately after having intercourse. Sperm can live for up to five days, while the egg has a short, sharp period of time when fertilisation can occur. Fertilisation itself is painless, microscopic and occurs silently within the fallopian tubes. It is completely out of the conscious control of any woman, no matter how much in control of her life she likes to think she is.

However, it is true that some women are absolutely certain they are aware of the moment when an embryo has embedded in their uterus. Remember, this generally occurs a few days after conception – not at the time of the egg being fertilised or having sex. If a woman has been charting her cycles, taking her temperature and is fairly confident that she knows when she has ovulated, then estimating when fertilisation occurred can be much easier.

Fertilisation usually happens within a window of time, 12-24 hours after ovulation, when the egg is viable and capable of being fertilised. Most women conceive within days 11-21 after the first day of their last period.

What’s the Luteal Phase?

The Luteal Phase is an important concept to understand when trying to conceive. This is the phase of your menstrual cycle which begins after you have ovulated and lasts until either pregnancy occurs or there is a breakdown in the corpus luteum and the period starts. The typical length for a luteal phase is 10 – 16 days, though most commonly it lasts for around 14 days. So, if you normally have a 28 day cycle, you are likely to ovulate on day 14; if you have a 30 day cycle, you are likely to ovulate on day 16.

Other Ways to Check Due Date

  • Ultrasound offers an accurate means of checking the baby’s growth and size. Vaginal ultrasound can be more precise than abdominal ultrasound. Many obstetricians have portable ultrasound technology in their consulting rooms.
  • The baby’s growth, though this is not always a good measure. Some babies are large, some small – there is no absolute “one size fits all”.
  • Palpating a mother’s abdomen and comparing the size of her uterus with classic size markers during pregnancy. For example, by 12 weeks the uterus is generally starting to lift out of the bony pelvis and can be felt just above the symphysis pubis. By 20 weeks, the uterus can be felt at the level of the mother’s umbilicus. Though again, these measurements are general guides, not exact.
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