Estimate Date of Conception

If you know the date of your last menstrual cycle, you can figure out possible dates of conception using a calendar. If you don’t know this, a doctor can estimate the gestational age with an ultrasound or through a physical exam. All estimates, even by doctors, are “best guesses” and cannot tell you the actual day you conceived, due to varying growth rates of babies and differences in women’s individual menstrual cycles.

Steps

Estimating Using Your Menstrual Cycle

1. Mark the first day of your last menstrual cycle on a calendar. Most women have a 28-day cycle, which means they start bleeding every 28 days. The first day of bleeding is the date your cycle begins.
   • If you do not know at least a general date of your last cycle, you cannot find your date of conception without visiting a doctor.
   • Cycles can be as short as 24 days or as long as 34 days in some women. Knowing the length of your cycle will be important when calculating ovulation.
2. Estimate date of ovulation at the middle of your cycle. You can only conceive when ovulating, but without daily testing for ovulation, there is no way to know for sure when it occurred. Doctors usually give a short range of dates surrounding the middle day of your cycle. If your cycle starts every 28 days, count backwards 14 days from the date bleeding began and mark it on the calendar.
   • Women with a regular 28 day cycle can ovulate anywhere between the day 11 and day 21, but day 14 is the standard for estimation.
   • If your cycle is 24 days, estimate ovulation at day 12. If your cycle is 30 days, ovulation probably occurred around day 16.
   • If your cycle varies month to month, use the average to calculate your date of ovulation. For example, if one month your cycle starts again after 24 days, but the next month it lasts for 26, use 25 days as your cycle length.
   • Once the egg is released from the ovary it has a roughly 24-hour lifespan in which it can be fertilized.
3. Note which days you were sexually active around ovulation. Since exact time of ovulation cannot be specified, knowing what days you had sex around the middle of your cycle can help you figure out when you conceived. Look at the week around your estimated ovulation date, and mark any in which you know you had intercourse.
   • Conception may have occurred on any of dates you had intercourse between day 11 and day 21 of your cycle depending on your date of ovulation.
   • Although you cannot get pregnant unless you are ovulating, sperm can live inside the body for up to five days. If you had intercourse in the five days before your estimated date of ovulation, it is possible sperm were still present when ovulation began and fertilized the egg.

Figuring Out Your Ovulation Cycle

1. Take your basal body temperature. If you are trying to figure out exactly when you start ovulating so you can become pregnant, taking your temperature daily can help since some women have a slightly higher temperature when they ovulate — about 0.2 to 1°F higher. Using a basal thermometer, take your temperature every morning before getting out of bed. Plot your temperature on a calendar for a month or two and look for a pattern.
   • You are most fertile two to three days before ovulation begins, so count the days between the first day of your last cycle and the day your temperature increased. Use this number to figure out your likely next date of ovulation, and count back three days to find the best time to engage in intercourse.
   • If your cycle is different from month-to-month, you may need to track for several months to find an average or a bigger pattern to know your likely date of ovulation.
   • Be sure to use a thermometer specifically designed to find basal temperature, or your body’s resting temperature. Also take it as soon as you wake up, before you begin moving around. Keep your thermometer on your nightstand to make it easy to reach from bed.
2. Monitor any vaginal secretions. Some women experience an increase in clear vaginal secretions, resembling raw egg whites, right before ovulation. It might be difficult to notice unless you are looking for it and comparing secretions every day.
   • Mark dates on a calendar that you notice any secretions. Use one symbol or color for any you consider “normal,” meaning frequently the same amount, color and texture, and any days you notice more or different secretions.
   • If there is a day or two you when have higher secretion levels or they resemble egg whites, assume your date of ovulation is the next day or so after that. Use this information to decide when it is best to have sex over the coming month.
3. Notice mild abdominal cramping. Some women experience mild cramping when they ovulate. If you can’t find your basal temperature, or don’t show a pattern of increasing temperature around the same time every month, start charting any abdominal discomfort you feel. There may be a pattern here to show when you usually ovulate.
   • For a month, mark every day on a calendar that you have any abdominal discomfort. Afterwards, look over the dates. If there were a couple days in the middle of your cycle during which you had mild cramping, this is likely when you ovulated.
   • Use the number of days in-between your last period and cramping to determine the best days to have sex in the coming month.
4. Buy an ovulation kit. Available as an over-the-counter test, this kit helps you monitor your urine for an increase in hormones that occurs most often right before ovulation. As soon as you are alerted to an increase, have sex daily until the hormones decrease again.
5. Test for pregnancy after you miss your period. Pregnancy begins when a fertilized egg starts to grow and attaches to the lining of your uterus. This can happen six days after fertilization (or your date of ovulation) and may take up to four days to complete. The best time to start testing is a few days after your period would normally start.
   • No matter the length of your menstrual cycle, do not start testing until you have missed your period. If you test any earlier you are more likely to get a false negative.
   • A negative test doesn’t mean you aren’t capable of getting pregnant, or that an egg wasn’t fertilized, just that it didn’t finish attaching before your menstrual cycle progressed.

Seeing a Doctor to Determine Gestational Age

1. Schedule an ultrasound. A doctor can determine the baby’s gestational age with an ultrasound by measuring the either the fetus' gestational sac, or the embryo or fetus, depending on stage of the pregnancy. Gestational age, considered the age of the baby, is determined based on the last menstrual cycle rather than conception. Subtracting two weeks from the gestational age will therefore give you a likely time of conception.
   • Gestational age (the age of the baby) can be measured by ultrasound as early as five or six weeks after your last menstrual cycle, and is easier to estimate between weeks eight and 18, since most babies grow at the same rate during this time. Your doctor takes measurements to determine how old the baby is based on size.
   • Estimating conception with the help of ultrasound is not foolproof. If you know the date of your last cycle and the gestational date determined by ultrasound is less than a week different, doctors still use your cycle date to predict when the baby will be born.
   • An embryo may develop a heartbeat as early as six weeks after fertilization and the heart is formed by week eight. During the second month of pregnancy, symptoms such as breast tenderness, nausea and vomiting, heartburn, and frequent urination begin. See a doctor as soon as possible if you notice one or more of these symptoms to best estimate your date of conception with an ultrasound.
2. Ask for a physical exam. If you do not know when your cycle started and don’t have access to or can’t get an ultrasound, a doctor can determine length of pregnancy around the 12th week by detecting if the uterus has grown over the pubic bone.
   • This is the least reliable method and is only used to determine gestational age if the other two aren’t available.

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