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In the previous blog post, we looked at the menstrual cycle and the fertile window.  Keeping these things in mind, let’s take a look at the difference between gestational age and the date of conception.

When a woman is pregnant, doctors, nurses, and midwives routinely ask about the first day of her last period (FDLP).  Why is this?  Well, in the past, the FDLP was the most reliable way to determine how far along in a pregnancy a woman was.  The number of weeks and days since her FDLP is how the pregnancy was dated, and this was relayed to the patient as the estimated gestational age (EGA).  Using this type of dating, a pregnancy carried to full term will last 40 weeks.

Looking at the menstrual cycle though, it is highly unlikely that a woman would get pregnant on the first day of her last period.  Even though this is true, in the past, the FDLP was a concrete date that could be used not only to estimate how far along she was, but also when she would deliver.  Since the introduction of routine ultrasound use in pregnancy, however, it has been discovered that the FDLP can over- or under-estimate how far along a woman is by about a week.  Additionally, some women don’t track their periods closely, or their cycles could be irregular due to recent childbirth or breastfeeding, PCOS, or the use of hormonal birth control, and the FDLP reported may be a general estimate.  Currently, early ultrasound is considered the gold-standard for dating a pregnancy.

Let’s give a brief example. Say three women came to a pregnancy clinic on July 21.  All three women report that their FDLP was June 1^st.  The first woman has the “text-book” cycle length of 28 days.  Assuming that this cycle was normal for her, she will most likely be 7 weeks 1 day pregnant by gestational age, and her baby would have been conceived about 5 weeks ago.  The second woman has a cycle length of 21 days.  Again, assuming this last cycle was normal for her, her gestational age could be 8 weeks 1 day, with her baby conceived about 6 weeks prior.  Finally, the third woman reports a 35-day cycle.  Her gestational age might only be 6 weeks 1 day, with conception occurring about 4 weeks prior.  All three women report the same FDLP, but they all could be at slightly different stages in pregnancy.

If there is such variation in pregnancy dating based off the FDLP, and if ultrasounds are the most accurate way to date a pregnancy, then why is the EGA still used when it’s based off of the FDLP?  In part, it probably is related to the fact that it is how pregnancy has been measured for generations.  But additionally, some women don’t receive an ultrasound in the early weeks of pregnancy, and once a woman is beyond 10-12 weeks EGA, the measurements are less precise and the FDLP becomes a more reliable way to date her pregnancy.  Unless the difference in EGA is substantial, if there is a difference in the date given by an U/S and a woman’s reported FDLP, the FDLP is considered more accurate.

Even though the guesswork in determining conception can’t be removed entirely, with a basic understanding of the menstrual cycle, the fertile window, and how gestational age works, it can be a lot less confusing.  While I learned the basics about the menstrual cycle in school, it wasn’t until after I had graduated that I learned more about the fertile window and the specifics of when conception could occur.  As women, it is good for us to understand the design of our bodies.  It can help us understand how to plan or prevent pregnancies without the use of hormones, if that is important to us, and it can help us recognize early if something unusual is going on that could warrant a visit to the doctor.

As a nurse, I have been asked by multiple women what the “conception date” of their baby is.  This can be a confusing topic, especially if a pregnancy is dated at, say, six weeks, but she only reports having sex once about four and a half weeks ago.  So how can one determine when a baby was conceived?

First, we need to review a few things about a woman’s menstrual cycle.  Over a period of about 21-35 days, the hormones in a woman’s body fluctuate.  The menstrual cycle begins on the first day of her period.  Estrogen and progesterone levels decline, and the lining of the uterus sloughs off.  Bleeding can last anywhere from 2-8 days, with some women reporting shorter or lighter periods, and others longer or heavier ones.  As the uterine lining is shed, hormones start changing again, with estrogen levels rising, causing the lining of the uterus to thicken and an egg in the ovary to mature.  How quickly this happens after a period starts depends on each individual person.

Once the egg is ready to be released, there is a surge of hormones that cause the egg to leave the ovary and enter the fallopian tube.  This is called ovulation, and it is accompanied by a rise in the hormone progesterone.  At this point, the egg is ready to be fertilized.  If it is fertilized, it travels down the tube and implants in the uterus about a week after conception. The high levels of progesterone make sure the uterine lining is ready for this implantation.  But if fertilization does not occur, the egg disintegrates and is reabsorbed into the body.  About two weeks after the egg is released, the cycle ends, and a new cycle begins with the starting of another menstrual period.

The phases of the menstrual cycle are standard to all women of childbearing age, though exceptions do occur, such as with PCOS (polycystic ovarian syndrome), and changes that occur naturally as a woman is nearing menopause.  Additionally, there are typically about 14 days between when ovulation occurs and the start of her next period, with usually only a few days variation between women.  What varies most often is the length of time during the first half of the menstrual cycle.

For example, if a woman’s cycle is only 21 days long, she most likely ovulated around day 7 of her cycle.  On the other hand, if a woman’s cycle was 35 days, she probably ovulated closer to day 20.

The difference in when a woman ovulates makes a difference in when she is considered fertile.  The “fertile window” is the days during a menstrual cycle that a woman could potentially get pregnant if she is sexually active.  The fertile window is often considered to be 5 days before through 48 hours after ovulation.  There are two factors that are considered when identifying the “fertile window”.  First, sperm can stay alive in a woman’s body for up to 5 days, so even if a woman doesn’t have sex on her ovulation day, if she did within the previous 5 days, she could still conceive.  Additionally, the egg can stay alive for 24-48 hours after ovulation, so she is still able to conceive during the day or two after she ovulates.