We get a lot of questions about fetal development at ACPC Women’s Clinic. Our patients are usually anxious to figure out their due date and to see what their baby actually looks like. A woman is usually about 4 weeks along before she even realizes she is pregnant. There can be some confusion around determining the due date. Here’s why:
Most women are surprised when the nurse says she is already 4 or 5 weeks along even though she had just missed her period within the last couple of days. This is because obstetricians count weeks of pregnancy from the first day of a woman’s last period (LMP – Last Menstrual Period). They do this because there is no way to determine exactly when conception occurred. To determine the age of your unborn child since fertilization (conception), use this table and just subtract two weeks.
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The most accurate way to determine how far along you are is to have an ultrasound. During an ultrasound at ACPC Women’s Clinic, one of our nurses will take measurements to determine the gestational age of the baby. The first and earliest ultrasound is usually the most accurate of ultrasounds throughout your pregnancy in determining gestational age.
At fertilization, also called conception, the sperm joins with ovum (egg) to form a single cell that contains the complex genetic blueprint for every detail of human development. At this point the child’s sex, hair and eye color, height, and skin tone are already determined. The unborn child at this stage is called a zygote until it reaches the uterus 3-4 days later, at which point it is called an embryo. On the 7th day after fertilization, the embryo finds a suitable place to rest and implants into the uterus.
Sources: A Child Is Born by Lennart Nilsson; Nine Month Miracle, A.D.A.M. Software, Inc; Introduction to Child Development, 6th ed., by John Dworetzky; More information about how fetal photographs were taken.