When Can You Find Out How Many Weeks Pregnant You Are?

Reviewed on 11/21/2020

When can you find out how many weeks pregnant you are?

A woman can find out how many weeks pregnant she is at any time during pregnancy.
A woman can find out how many weeks pregnant she is at any time during pregnancy.

A woman can find out how many weeks pregnant she is at any time during pregnancy. There are several ways to calculate how many weeks pregnant a woman is and the estimated due date or estimated date of delivery (EDD). Some ways are more accurate than others. It may be calculated based on the woman’s last menstrual period (LMP), her urine levels of the pregnancy hormone human chorionic gonadotropin (hCG) , fetal developmental milestones and an ultrasound. The due date is just an estimate and only about four percent of women give birth on their due date. Most women either deliver a little earlier or a little later than their estimated due date, which is normal because childbirth depends on several factors related to the mother and her fetus.

What are the different ways to estimate the due date of a pregnancy?

The following ways help calculate how many weeks pregnant a woman is and the estimated due date.

Calculating by using the date of the last menstrual period (LMP)

This is one of the most common, simple and most accurate ways to figure out how many weeks pregnant a woman is and estimate the approximate due date. The way to calculate the due date is to take the first date of the last normal period and add 280 days (40 weeks), which is the typical length of a normal pregnancy. The period that is to be tracked should be typical of a woman’s menstrual cycle. This means it should be the same number of days as a usual period, the same amount of bleeding and the same amount of discomfort. That’s because sometimes a woman may bleed after she’s pregnant, but that bleeding is different from a normal period.

  • The formula for calculating how many weeks pregnant a woman is the number of days since the first day of her last menstrual period (LMP) divided by seven. So, the number of days since the start of LMP divided by seven equals the number of weeks pregnant
  • The formula to calculate how many months pregnant a woman is the number of days since the first day of a woman’s last menstrual period divided by 30. This means the number of days since the start of LMP divided by 30 equals the number of months pregnant.

To be able to calculate how many weeks pregnant a woman is, she has to keep track of her menstrual period. The reason the calculation is made based on the first day of the last period is that the physiology of ovulation was not clearly understood in times past. Ovulation occurs only a couple of weeks after the onset of a woman’s period, so the only information doctors had was the date of her period and it was found that most women gave birth around 280 days after the first day of the last menstrual period. Hence, all research, medical literature and clinical practice were based on that proven formula.

If the woman hasn’t been keeping track of her periods or has irregular periods, it may not be possible to accurately estimate how many weeks pregnant she is or the due date. Hence, a dating ultrasound would be advised as early as six to seven weeks.

Calculating using a dating ultrasound

During a dating ultrasound, the length of the fetus from crown to backside is measured to accurately estimate the due date or how far along the pregnancy of the woman is. It is performed within the first trimester (the first 13 weeks of pregnancy). After the first 13 weeks, the growth rate of the fetus varies, hence the estimation of the number of weeks pregnant the woman is or the due date may not be as accurate to predict with an ultrasound. Even if the LMP is known, an obstetric ultrasound is still advised every three to four weeks to monitor growth rate and assess fetal health.

Calculating using a pregnancy test with a “weeks indicator”

All pregnancy tests work by detecting the pregnancy hormone, human chorionic gonadotropin (hCG), in the urine. However, a “weeks indicator” claims to tell the number of weeks pregnant the woman is, but the results are vague and only give approximate figures like one to two weeks, two to three weeks or three or more weeks pregnant. These tests measure from the time of implantation (when the embryo attaches to the uterus) and not from the LMP. A test might say that the woman is one to two weeks pregnant when she is actually four weeks pregnant based on pregnancy calculations.

Calculating based on fetal milestones

In the past, the first heartbeat and first movement were used to get a sense of how many weeks pregnant a woman is and the age of the fetus. A mother usually feels the fetal movements at about 20 weeks of pregnancy,   but this is not a rule. These methods are not used anymore because they are not accurate.

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References
Medscape Medical Reference

Perinatology.com


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