Read more about ultrasound on this page or our due date page. Pregnancy is counted from a women's last menstrual period because the start of a period is easily observable and usually known to a woman whereas the date of conception is much less likely to be known. Historically, the first day of the last menstrual period LMP became the date from which pregnancy is counted even though conception occurs about two weeks later.
The LMP formula is good for working out an initial age of pregnancy but ultrasound is more accurate. Ultrasound in the first trimester is the most accurate method of dating pregnancy. If there is a significant difference between gestational age according to LMP and gestational age according to the ultrasound, you may be given a new gestational age and due date based on the ultrasound measurements.
Your pregnancy may also be dated by ultrasound measurement if an LMP formula was not appropriate for your pregnancy for any reason. LMP formulas are not suitable in several situations including the following: LMP is not known or is not certain, birth control pills were taken in the two months before LMP, the menstrual cycle is irregular, you are carrying multiples twins or more.
Although the age of pregnancy based on ultrasound measurements is more accurate than the age of pregnancy based on LMP, both are estimates and not definitive statements.
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This Internet site provides information of a general nature and is designed for educational purposes only. If you have any concerns about your own health or the health of your child, you should always consult with a doctor or other healthcare professional.
Please review the Terms of Use before using this site. In this post, we will tell you how to calculate pregnancy weeks and months. Read on! Your pregnancy due date is calculated by adding 40 weeks i. This is done with an assumption that you have a day menstrual cycle. The time between your menstrual period and ovulation is calculated as the first two weeks of pregnancy, as ovulation takes place two weeks after the first day of LMP.
If you deliver the baby on the estimated due date, she is 38 weeks old after conception and not 40 1. If you miss your first period, you are likely to be four weeks pregnant including two weeks of time from menstruation to ovulation 2.
Pregnancy is calculated based on your last menstrual period. Week one of your pregnancy will start on the first day of your LMP.
Likewise, the following weeks are calculated. This may sound weird, but it helps in giving the exact date for timely medical care. Unless you had an assisted conception and knew when the fertilization took place, the LMP method is the only way to know the closest ovulation and conception dates 3.
The doctors can help you with the exact calculation of weeks and months of pregnancy through other ways. Doctors and midwives use a standard set of three methods to count pregnancy weeks — based on LMP, using ultrasound, and through physical examination. Though the actual timing of a delivery cannot be known, the three-step process helps you predict the likely date of birth.
Around 12 weeks, your medical practitioner can feel the uterus fundus above the pelvis. After 18 weeks, the distance in cm between the fundus and pubic bone will be the same as the number of weeks from the last menstrual period. In about 20 weeks, the fundus is at the umbilicus.
This gives a rough idea about the pregnancy week. However, this is not always the right way to predict the gestational age due to factors such as twin pregnancy, obesity, uterine tumors, or the fundus being higher or lower 4. Ultrasound works even when you are not aware of the LMP or if your periods are irregular. It is based on a series of measurements of the gestational sac and the fetus.
The sonographer measures the baby from head to toe. The educational health content on What To Expect is reviewed by our medical review board and team of experts to be up-to-date and in line with the latest evidence-based medical information and accepted health guidelines, including the medically reviewed What to Expect books by Heidi Murkoff.
This educational content is not medical or diagnostic advice. Use of this site is subject to our terms of use and privacy policy. Registry Builder New. Due Date Calculator When is your baby due? Use this pregnancy calculator to find your due date based on the date of your last menstrual period, conception date, IVF 3-day or 5-day transfer date or date of your last ultrasound. Calculation Method. The first day of your last period.
Date of Conception. Date of Transfer. Date of Ultrasound.
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