How to Calculate Pregnancy Weeks: Your Cycle and Symptoms

How to calculate pregnancy weeks?

Hello Doctor, my last menstrual period was from February 24 to February 28, and my next period was expected to start on March 24, but it has been delayed by three days.
I had sexual intercourse on March 23 and 26.
I went to the gynecologist for an examination, and both the ultrasound and urine tests showed that I am not pregnant.
The doctor mentioned that a pregnancy test would only show positive results four weeks after conception, and advised me to take another test next week.
Does "four weeks" refer to four weeks after the sexual intercourse? Additionally, last week I experienced gastrointestinal discomfort, had diarrhea for two days, vomited once, and had a headache for two days.
I visited my family doctor and took medication for enterovirus.
Now I am concerned whether my symptoms are due to gastroenteritis or early pregnancy.
This week my body feels normal, and I have no discomfort.

Niming, 20~29 year old female. Ask Date: 2019/03/27

Dr. Huang Jianzhong reply Obstetrics and Gynecology

The last menstrual period was from February 24 to February 28, and the next period was expected to start on March 24, which has been delayed by three days.
Sexual intercourse occurred on March 23 and 26.
A visit to the obstetrician-gynecologist included an ultrasound and urine test, both of which indicated no pregnancy.
The doctor mentioned that a urine test would only show pregnancy after four weeks, and advised me to return for another test next week.
Does the "four weeks" refer to four weeks after sexual intercourse?
Answer: The statement that a urine test will show pregnancy after four weeks refers to the time after the missed period or one week after the period is overdue.
Additionally, I experienced gastrointestinal discomfort last week, with diarrhea for two days, vomiting once, and a headache for two days.
I consulted a family physician and took medication for viral gastroenteritis.
Currently, I am feeling normal with no discomfort.
I am concerned whether my symptoms were due to gastroenteritis or early pregnancy.

Reply Date: 2019/03/27

More Info

To calculate pregnancy weeks accurately, it is essential to understand the standard method used in obstetrics, which is based on the first day of your last menstrual period (LMP). This method is widely accepted because it provides a consistent starting point for determining gestational age, even though conception typically occurs about two weeks after the LMP in a regular 28-day cycle.

In your case, if your last menstrual period started on February 24 and lasted until February 28, you would count from February 24 as the first day of your last period. If your next period was expected around March 24 and has been delayed by three days, you would be considered approximately 5 weeks pregnant (counting from February 24).
Regarding the confusion about the four-week wait for a pregnancy test to show positive results, this generally refers to the time after conception, not the date of sexual intercourse. Since ovulation typically occurs about 14 days after the first day of your last period in a regular cycle, the four-week mark is usually calculated from the LMP. Therefore, if you had sexual intercourse on March 23 and 26, and if ovulation occurred around March 10 (assuming a typical cycle), it would take about a week after the missed period for a home pregnancy test to detect the hormone hCG (human chorionic gonadotropin) in your urine.

As for your symptoms of gastrointestinal discomfort, diarrhea, nausea, and headaches, these can be caused by various factors, including viral infections, dietary indiscretions, or even early pregnancy symptoms. Early pregnancy can sometimes cause gastrointestinal symptoms due to hormonal changes, but it is also possible that these symptoms are unrelated to pregnancy and are instead due to a stomach virus or other gastrointestinal issues.
If you are feeling better now and have no further symptoms, it may be a good sign. However, if you continue to have concerns about your health or the possibility of being pregnant, it is advisable to follow up with your healthcare provider. They may recommend a blood test for pregnancy, which can detect hCG earlier than a urine test and provide more definitive results.

In summary, to calculate your pregnancy weeks, start from the first day of your last menstrual period. If you are unsure about your pregnancy status, waiting a week and retesting or consulting with your doctor for a blood test can provide clarity. If you experience any unusual symptoms or have concerns about your health, do not hesitate to seek medical advice. It is always better to be cautious, especially when it comes to your health and potential pregnancy.

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