Due Date Calculator
Your menstrual cycle can generally be described as having two distinct phases – the follicular phase and the luteal phase. The follicular phase begins on the first day of your period (cycle day 1) and ends at ovulation. The luteal phase begins after ovulation and ends at the start of your menstrual period, which marks the beginning of the next cycle.
During the follicular phase, follicles in your ovaries mature and compete with each other in preparation for ovulation. At the time of ovulation, the dominant follicle ruptures and releases a single egg cell from the ovary. The egg will then travel down your fallopian tube where it becomes available for fertilization by sperm. If not fertilized, the egg will degenerate within 12 to 24 hours after ovulation. Thus, fertilization must occur shortly after ovulation, in order to achieve pregnancy. While this may seem like a very short window of time, sperm can survive for several days within the woman’s body, and thus even sperm from intercourse a few days prior to ovulation may be able to fertilize the egg shortly after ovulation.
During the luteal phase, the ruptured follicle becomes a gland known as the corpus luteum (meaning “yellow body”). The corpus luteum secretes the hormone progesterone, which is responsible for maintaining the uterine lining to support a pregnancy. In the absence of fertilization, the corpus luteum degenerates, resulting in a drop in progesterone and the shedding of your uterine lining, marking the beginning of menses (your period). If fertilization does occur, the corpus luteum continues to produce progesterone, maintaining your uterine lining and aiding to support your pregnancy until the placenta is fully functional and able to take over its hormone-producing duties.
Most information available about fertility, pregnancy timing, and the reproductive cycle assumes an average cycle length of 28 days, with ovulation occurring on cycle day 14. While these numbers are indeed well accepted averages, significant variation can occur in the menstrual cycle length and timing of ovulation. However, most of this variation can be attributed to the follicular phase – the phase of the cycle before ovulation, when your body is preparing to ovulate. If you have irregular or longer menstrual cycles, it is most likely due to shorter or longer follicular phases. The follicular phase length is also more easily influenced by external factors such as stress, illness, and dietary changes.
The length of the luteal phase, which is the time after ovulation and before the start of the menstrual period, is actually quite predictable and rarely varies by more than a day. Most women have a luteal phase between 13 to 15 days, although anywhere between 10 to 16 days is considered “normal.” If your luteal phase is typically 14 days long, you may occasionally have a 13 or 15-day luteal phase, but rarely much more variation than that. Likewise, if you normally have a 12-day luteal phase, you can expect it will almost always last between 11 to 13 days. The exception to these statements is that the luteal phase does often shorten with age. A woman approaching 40 may have a slightly shorter luteal phase than she did in her 20s.
If you’ve ever kept track of your basal body temperature (BBT) for an extended period of time, you likely know the length of your average luteal phase. Even if you are not currently tracking your BBT, you can use this information in the fertility calculator or ovulation calculator that we provide on this page to achieve a more accurate prediction of your fertility status.
While most of the available pregnancy calculators, due date calculators, online pregnancy quizzes, and fertility/ovulation calculators assume an average 28 day cycle with ovulation on day 14, we’ve created a series of calculators that can be personalized to account for your individual norms. If you know your personal average cycle length, luteal phase length, or ovulation date, you will have the option to enter this information into the fertility, ovulation, or pregnancy calculator, if applicable. This enables the calculator to give you a more accurate prediction of your fertile window, time of ovulation or conception, estimated pregnancy due date, and/or gestational age (how far along you are in your pregnancy). If you do not have this information, the calculator will use the typical average values as a default (28 day cycle length with 14 day luteal phase). Read on for more information about natural symptoms of ovulations and using the individual calculators. Please note that, as with all online calculators, these calculators will be most accurate if you have regular cycles.
Signs of Ovulation
Combined with your natural symptoms of ovulation, the ovulation calculator and fertility calculator can help you reach your pregnancy goals. Different ovulation symptoms occur at different points around ovulation. If you are trying to conceive, perhaps the most important ovulation symptom is the increase in cervical fluid (vaginal discharge) that occurs as ovulation draws near. As you approach peak fertility, your cervical fluid will change in amount and consistency from a dry/sticky consistency, to a creamy/lotiony texture, to a very wet, stretchy, egg white-like consistency. This “egg white” fluid is the most fertile type of fluid, helping to create an ideal environment for sperm to survive and travel from the vagina, through the cervix, up the uterus, and up the fallopian tubes to meet with the egg. When using the fertility calculator, the estimated fertile window predicted by the calculator, should approximately line up with the time you have the most fertile fluid.
Ovulation predictor and fertility kits can also alert you to upcoming ovulation. Most of these kits function by detecting one of the invisible signs of ovulation: a release of Luteinizing Hormone (LH), which typically spikes shortly before ovulation. Your LH peak should also fall within your predicted fertile window.
While cervical fluid and ovulation predictor kits can be excellent indicators of upcoming ovulation, neither of these can confirm that ovulation has occurred. It is possible to experience these signs of ovulation without actually ovulating. Some women, especially those with polycystic ovary syndrome or those who are breastfeeding, may experience several phases of increased fertile fluid before ovulation actually takes place. Stress, hormonal imbalances, weight fluctuations, and dietary changes may also play a role.
Another symptom of ovulation is a marked increase in BBT resulting from rising progesterone levels after ovulation. Your BBT is detectably higher the morning after ovulation and will remain elevated until the end of your luteal phase. In contrast to tracking your cervical fluid, charting your BBT can let you know whether or not ovulation has taken place, but will not alert you of upcoming ovulation (since the temperature shift does not occur until after ovulation). By the time a temperature shift is detected, the ovulated egg may not be viable. Intercourse occurring at any point within the fertile window (about 5 days before ovulation), but especially on the two days before the temperature shift will have the best chance in resulting in conception.
While not as common, some women experience additional ovulation symptoms, such as light spotting (very light vaginal bleeding) for a day or two around ovulation. This spotting is not to be confused with implantation spotting that can occur after ovulation and is a very early sign of pregnancy. Others feel a dull ache or sharp twinge in their lower abdomen shortly before, during, or after ovulation. This ovulation pain is known and mittelschmerz (German for “middle pain”).
Visit the Fertility Awareness Center website for more information for how to track your symptoms of ovulation and other fertility signs, as well as how to use this valuable information. In addition, several websites and phone apps offer specialized software to help you chart your cycles and signs of fertility. Both Ovusoft and Fertility Friend offer free and paid services for charting your cycles.
Online Fertility and Pregnancy Calculators
Ovulation Calculator Breakdown
If you are trying to get pregnant, knowing your ovulation date may improve your chances of success. As previously mentioned, in order for pregnancy to result fertilization (the meeting of sperm and egg) must occur within 24 hours after ovulation, since the egg has a very short lifespan. Thus, you are most fertile the day before and the day of ovulation. If you know when to expect ovulation, you can more easily time intercourse for your most fertile time. For this reason, many women track their natural fertility signs and maintain an ovulation calendar to keep track of previous and predicted ovulation dates. You can use the ovulation calculator on this site to predict your approximate date of ovulation, based on the first day of your last period. If you know your average cycle length and/or luteal phase length, you can input this information to receive a more accurate prediction.
The ovulation calculator is designed to provide a simple, clear method to estimate your ovulation date. Whether or not you keep an up-to-date ovulation calendar, you can use this information for estimating when you last ovulated or predicting future ovulation dates. Since conception occurs shortly after ovulation, you can also use the ovulation calculator as a conception calculator for estimating the date your baby was conceived. In addition to your estimated due date, the pregnancy calculator on this page will also give you an approximate conception date, if you do not know it already.
You may also be wondering about the best time to take a pregnancy test. A home pregnancy test will be most accurate at least 14 days (2 weeks) after ovulation, even for the brands boasting “early results.” This is approximately the time when your period would be due. If you choose to test early, keep in mind that an early test is more likely to give a negative result even if you are pregnant. Also remember that unless you’ve confirmed ovulation through a basal body temperature shift or ultrasound, you may have ovulated earlier or later than predicted. Thus, if you get a negative result, you may find it necessary to re-test a week or two later if your period is still “missing.” Read our previous article for more details about when to take a pregnancy test.
Fertility Calculator Breakdown
The fertility calculator is designed to help you plan intercourse on your most fertile days. These would be the days when you would expect to have the most fertile cervical fluid, as discussed above. Whether or not you formally keep track of your fertility symptoms, the fertility calculator may be used to help you plan ahead and aid you in reaching your fertility goals. If you are aware of your fertility and ovulation symptoms, you can use this information to corroborate your predicted fertile window, and adjust your inputs for the next cycle, if needed. Similar to the ovulation calculator, the fertility calculator is based on the date of your last menstrual period with options to include your average cycle length, and average luteal phase length for a more accurate prediction. If you are unsure of your average cycle length or luteal phase length, then we will assume a 28-day cycle and 14-day luteal phase as defaults.
Due Date/Conception Calculator Breakdown
The due date calculator on this site gives you the option of estimating your due date using either the first day of your last menstrual period (LMP method/“Naegele's rule”) or your estimated date of conception/ovulation. Most often, your baby’s due date and how far along you are in your pregnancy (fetal gestational age) is calculated based on the first day of your last menstrual cycle, with the start of your period considered the beginning of pregnancy week 0, and ovulation assumed to occur at the start of week 2. This may seem confusing since fertilization cannot actually occur until week 2 of pregnancy, but this is the accepted, standardized method for pregnancy dating. As previously mentioned, it is best to wait until at least 2 weeks after ovulation before taking a home pregnancy test. If pregnant, this would be week 4 of pregnancy.
Due to the previously mentioned variations in cycle length and ovulation date, the LMP method may not be the most accurate method but will be fairly reliable if you typically have regular, 28-day cycles. If you have pretty good idea of when you ovulated based on BBT or ultrasound, entering your ovulation date in the due date calculator will likely provide the most accurate estimation of your due date. In addition to your estimated due date, this due date calculator also functions as a conception calculator, using the information you enter to give you an estimated conception date, and will also let you know approximately how far along you are in your pregnancy. A home pregnancy test will be most accurate if you wait until you are at least 4 weeks pregnant (2 weeks post-conception).
Too early to take a pregnancy test? A few women experience early pregnancy symptoms, such as implantation spotting, even before missing their period. Read more about early signs of pregnancy.
Please note that all due date estimations are always approximate, regardless of which method is used. It may be helpful to think of your due date as the center point of your “due month,” since healthy, at-term babies can be born several days to even a couple of weeks before or after their estimated due dates.
Your healthcare provider may prefer one due date estimation method over another. A first trimester ultrasound is currently considered the most accurate method for clinicians to estimate a due date. Thus, your health care provider may schedule an ultrasound to confirm your due date, especially if you commonly experience irregular cycles, or there is another suspected discrepancy.
We sincerely wish you the best in your journey to conception. We hope this article and the tools provided have been helpful to you.
Already pregnant? Congratulations! This might be a good time to read more about early pregnancy symptoms and what to expect as you move forward.