Any calculation of a conception date is an estimate, even in a woman with regular 28-day menstrual cycles. Except for in-vitro fertilization (IVF) and other assisted reproductive procedures, you cannot precisely calculate the exact day a sperm meets an egg and fertilization (conception) occurs. However, you can use various factors and knowledge about reproduction to help narrow down the date to within one to five days if your cycles are regular.
Factors That Determine Conception Date Accuracy
The accuracy of estimating your date of conception depends on several factors about your menstrual cycle and ovulation, as well as facts about the egg and the sperm. The day of conception depends on:
- The usual average length of a woman's menstrual cycle
- Having regular menstrual cycles
- Accuracy by which the day of ovulation can be pinpointed
- The timing of intercourse around ovulation
- The lifespan of the egg and sperm
An obvious fact is you cannot conceive until you ovulate; therefore, the most likely date of conception is on the day of ovulation, if there is sperm around. If you can determine that day, you will come close to identifying your date conception. There are various ways to estimate the day of ovulation.
Calculation By Menstrual Cycle Length
You can estimate your date of ovulation and conception from the length of your menstrual cycle if this is fairly regular. The most consistent part of the cycle is the second half after you ovulate. No matter how long or short the cycle, you will get your next period around 12 to 14 days later.
If your cycles are regular, to estimate the day you most likely ovulated:
- Take your most usual menstrual cycle length (which counts from day one of a previous period to day one of the next).
- Get out your calendar and check the date of the first day of your last period of your conception cycle.
- Count forward the days of your cycle length to get the date you expected your next period - but instead got pregnant.
- Count backwards 14 days from that date.
- Ovulation, and the likely day of conception, occurred the day before that date, or one or two days before or after.
Twenty-Eight Day Cycles
If you have a consistent 28-day menstrual cycle, you most likely ovulated and conceived on day 14. However, you could have conceived on day 12 or 13 if ovulation was earlier, or days 15 or 16 if later. So, by this example you can understand you have a five day window during which you could have conceived.
Other Menstrual Cycle Lengths
You can do a similar estimation for cycle lengths shorter or longer than 28 days. For example:
- If you have 24-day cycles, you will ovulate and conceive about day ten of your cycle.
- If your cycle days are 38 days long, you will ovulate and conceive about day 24 of your cycle.
Women who have irregular periods will have a more difficult time determining the date of conception using the menstrual cycle length method.
Other Ways to Estimate Date of Conception
If you were tracking other signs of fertility, such as basal body temperature charts, or ovulation predictor kits, they can help you narrow down your conception date to within one to two days.
If you were charting increased pain at mid-cycle, the day of the worse pain on one side of your pelvis is the likely day your egg ovulated. This is probably the day you conceived if you had intercourse around that time.
Ovulation Predictor Kit
An ovulation predictor kit (OPK or LH kit) reliably predicts the window of ovulation, according to a study published in 2000 in Human Reproduction. The at-home urine test measures the increase in the pituitary luteinizing hormone (LH) that occurs in the days leading up to ovulation. Your most fertile time to conceive is within the day or two after a positive result.
If you did an LH kit during your concepion cycle, take a look at your test results. The day of your definitive positive reading is the day of the biggest increase in your LH. This "LH surge" triggers ovulation and predicts it is likely to occur 24 to 36 hours later. That one to two day window after your surge is the likely time you conceived.
If you kept a basal body temperature chart (BBT) during the cycle you conceived, this is a useful tool to help pinpoint when you ovulated and therefore conceived. Here are some helpful facts:
- Your temperature is below 98 degrees Fahrenheit in the first half of your cycle, then shifts above 98 degrees the day after you ovulate.
- In an idealized 28-day cycle, this shift to a higher temperature usually occurs on day 14.
- It stays elevated if you conceive or falls below 98 the day you get your period.
- Identify the last day your BBT chart was below 98 degrees during the first half of your cycle. That is your day of ovulation, and your likely day of conception. Note your temperature might dip a few degrees on the day of ovulation before rising above 98 the day after.
- In a 28-day cycle, the day of ovulation/conception is usually about day 13 or 14.
The temperature shift occurs because of an increase in progesterone production after ovulation.
Cervical Mucus Changes
Keeping track of cervical mucus changes within a menstrual cycle can help narrow down the day of ovulation. If you tracked these changes, the day before your mucus changed to a thick, tacky, cloudy, dried-up mucus was the day of ovulation, and your likely date of conception. The day of ovulation, your mucus would have been at its most thin and watery, as well as clear and stretchy like egg white. This is most likely the day you conceived.
Your Estimated Due Date
Once your doctor or midwife gives you your due date, count 38 weeks back on a calendar to find your most likely conception date. This calculation assumes the following:
- Pregnancy lasts 40 weeks.
- Your menstrual cycles are 28 days long and regular.
- You ovulated and conceived on day 14 of your cycle.
- Your due date or gestational age of your baby is two weeks longer than the conception or fetal age.
If your cycles are longer or shorter than 28 days, this method is less accurate.
The Lifespan of the Egg and Sperm
It is important to remember the lifespan of the egg and sperm can influence the date of conception. According to Clinical Gynecologic Endocrinology and Infertility (page 1285), the egg survives in the female reproductive tract only for about 12 to 24 hours after ovulation. Sperm retains its best ability to fertilize an egg for an average of three days in the tract after ejaculation, although it can survive up to five to seven days. By these facts:
- There may be viable sperm still present to fertilize your egg if you have sex only once up to three to five days before you ovulate. In this case, your day of conception will most likely be on the day of ovulation, even if you don't have sex that day.
- If you don't have intercourse until the day after ovulation, you may catch and fertilize a still viable egg. In this case your conception day will be one day after you think you ovulated.
A Conception Date Calculator
A conception date calculator can make it easier to figure out your date. The calculator below works for you if your menstrual cycles are regular and are between 24 and 38 days long. If your cycle lengths vary a lot, your results might not be accurate.
The calculator assumes ovulation occurs 14 days before the date of an expected period as explained in the section on 'Calculation by Menstrual Cycle Length' above. To calculate your conception date:
- Enter the average length of your menstrual cycles in the first box.
- Enter the month, day, and year of the first day of your last period in the next boxes.
- Click the 'Calculate Date of Conception' bar.
The calculator will display the date you likely ovulated and conceived.
Conception Date and Pregnancy Dating
Methods used to estimate conception date rely on trying to identify the day of ovulation. The various methods are accurate to within one to five days. It is important to note doctors don't use the date of conception to date a pregnancy. Instead, they date a pregnancy and estimate a due date from the date of the last menstrual period or by an early pregnancy ultrasound.