Estimating Conception Date

Estimating conception date for your pregnancy may be important to you, especially if it was unplanned. Whether you want to know if it was the “wine and flowers” date night or the "hot-and-heavy" late night, conception dates can help determine due dates.

LMP Method of Estimating Conception Date

The LMP (last menstrual period) method of estimating when you conceived is how most doctors figure your conception date and due date. For women who have a regular 28-day cycle, ovulation often occurs on or around Day 14.

Most women assume that their conception occurred around mid-cycle. However, if you have irregular cycles, you can use other methods of determining conception dates.

Ultrasound Estimates

Ultrasounds are usually performed at some point between weeks eight and 20 of pregnancy. They are used to confirm gestational age of a pregnancy. Gestational age is based on your last missed period. The age of the fetus (known as fetal age) is the age of your child since conception. Gestational age should always be one to two weeks older than fetal age.

To figure out the gestational age of the child, doctors and radiologists take measurements of the baby’s legs, arms, head, and torso. Based on these measurements, doctors can give an estimated gestational age of your child. Subtract two weeks to find the fetal age, and then count back in the calendar to figure out your estimated conception date.

Ultrasounds can only provide estimates of gestational age. Some babies are smaller or larger than the average. Earlier ultrasounds tend to be more accurate. Overall, they can be up to two weeks off on estimation.

Conception Dates are Only Estimates

Conception dates are only considered estimates for several reasons. First of all, even regular cycle women may not ovulate on Day 14 each month. It can vary slightly, anywhere from Day 12 to Day 16 for a regular woman on a 28-day cycle. Other reasons conception dates are just estimated include:

• Sperm life can be up to three to five days.
• Stress or other life changes can change even regular women’s cycles for that month.
• Eggs take 12 to 24 hours to be shed, so the estimated conception date may be off by one day.
• Spotting at implantation can be mistaken for a last missed period, throwing off conception date by as much as a month.

If you have been keeping track of your basal body temperature, you may know the length of your luteal phase--otherwise known as days past ovulation. A slight rise in the body temperature indicates that ovulation has occurred. The days between ovulation and the day before your next period are considered the luteal phase. Even if cycles are irregular, DPO usually stays the same. Therefore, you can count back using your DPO to begin estimating conception date.

The only completely accurate way of knowing conception date is when infertility treatments, such as artificial insemination or in vitro fertilization treatments have been used.

If you have had multiple partners in the two weeks surrounding your mid-cycle, you should consider getting a paternity test.

Conception Date Calculates Due Date

If your period is irregular but you can kind of estimate conception date, you may be able to calculate an estimated due date for your baby. Pregnancies average about 40 weeks long. However, to figure your due date based on conception date, use 38 weeks as your guide. That is because due dates are figured using gestational age, not fetal age.

Online calculators can help estimate both dates. Women who chart their cycles may also be able to figure their conception and due dates.

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