Your Last Period - Why The Date Is Important To A Sonographer
Written by Joy Biradee
July 13, 2021
If you are a female, a sonographer may have asked you when you saw your last menstrual period, or simply, when you had your last period. You may have wondered why they asked such a question when facing a machine or a screen. This article explains why.
But before this explanation, let’s have a very brief summary of the menstrual cycle.
The first day of your cycle actually begins with day 1 of your period. That day is very important to sonographers, so keep that in mind. Your period runs anywhere between 3 and 7 days. Meanwhile, the hormones responsible for ovulation are at work already, and your uterus (or womb), sheds its lining because you didn’t get pregnant during the previous cycle. The womb begins to prepare for another cycle.
The lining of the womb (called the endometrial lining) begins to be replaced almost immediately after the period stops. The purpose of this inning is to nourish a fertilized egg until it grows to a certain size. So this lining gets thicker and thicker as ovulation approaches and gets even thicker post-ovulation. One day, it realizes that the egg released during ovulation was not fertilized and the body prepares to shed the lining it had built.
As a new cycle begins with the shedding and regrowing of the lining, changes occur in your womb and ovaries. For example, your ovaries are empty after ovulation, but they begin to develop follicles at the start of your cycle. These follicles will get many and wither off except for 1 or 2 that reach ovulation. [1, 5]
As mentioned above, the changes we have discussed signify at what time a woman’s cycle may be. However, sometimes due to a hormonal imbalance or a disease, one may see the uterus looking like it shows a phase in the cycle when it is not a true picture. Below, is a little bit more information, but note that this list is by no means exhaustive. The focus will be on the more common abnormal findings.
In this case, the lining of the uterus becomes too thick for the menstrual phase. For example, if you have just finished your period within the last 48 hours, the sonographer or sonologist will expect to see an almost non-existent endometrial lining. But in endometrial hyperplasia, it looks as thick as, or even thicker than one would expect to see the day before your period. The easiest way to rule out this condition is to know the day of your last period. This diagnosis is significant especially in recognizing signs of disease and most possibly, uterine cancer. 
This is usually a result of hormonal issues, menopause, decreased fertility in general, or a disease process such as Pelvic Inflammatory Disease (PID). 
The pouch of Douglas or Posterior cul-de-sac is a blind end just behind the womb. It is usually empty except for a few times:
This occurs when ovulation does not occur during a cycle. The woman may still have a period despite this. When this happens, it is called an Anovulatory Period. So when a scan is done and no follicles are visible , it may be because ovulation has just occurred or because there was a failure to ovulate. We can diagnose this with a combination of our knowledge of what we see and the feedback we get from you and this includes “that question” - what was the date of your last menstrual period?
This usually occurs as a result of an underlying disease such as hypothyroidism. If the cysts are of a certain size, it is easy to miss out on an early diagnosis if the sonographer is unsure if these are cysts or just follicles in the process of development. Knowing at what stage the patient is in their cycle will be of help.
When an ultrasound specialist scans and sees a fetus in the uterus, they are required to date it, the age of the fetus is determined by confirming the date of the last menstrual period and comparing it with the current size of the fetus. Once a date is set, other scans help ensure that the baby is growing properly.
If the difference between the size of the baby seen and the date that is calculated from the last menstrual period is significantly different, it may spell trouble for the baby and/or mother. If the baby is too small compared to the age calculated from the period, it may mean growth retardation, and this requires specialist care. If on the other hand, the fetus is too big, it may spell trouble for the mother as she may have gestational diabetes and requires specialist care. 
The reasons a sonographer wants to know when you saw your last period have been clearly explained. The date of your last period is an important one to keep, so all women are encouraged to have a personal calendar or use an app on their phones to track their period.
1. National Center for Biotechnology Information - The Normal Menstrual Cycle and the Control of Ovulation
2. Radiopedia.org, the wiki-based collaborative Radiology resource - Endometrial Hyperplasia
3. Medical News Today - Endometrial Thickness: What is normal and how to measure
4. Radiopedia.org, the wiki-based collaborative Radiology resource - Differential diagnosis of free fluid in cul de sac (pouch of Douglas)
5. Radiology Key - Ultrasound in Follicle Monitoring for Ovulation Induction/IUI
6. National Center for Biotechnology Information - Pregnancy Dating