Weeks 3-4 - Gestational Age (Fetal Age 2 weeks)
The earliest change that can be seen through a vaginal ultrasound at
this time will be the “decidual reaction” which is the thickening of the endometrium. The endometrium lining
thickens as the blastocyst burrows into it. This cannot always be detected by ultrasound—sometimes it may take a
special eye or very good equipment to see this “reaction” in the endometrium lining.
*A key fact to remember when using ultrasounds is that a transvaginal ultrasound can detect
development in the uterus about a week earlier than a transabdominal ultrasound.
hCG: Once implantation occurs, the pregnancy hormone Human Chorionic Gonadotropin (hCG) will
develop and begin to rise. This hormone will signal that you are pregnant on a pregnancy test. hCG can be detected
through two different types of blood tests or through a urine test. A quantitative blood test measures the exact
amount of hCG in the blood, and a qualitative hCG blood test gives a simple yes or no answer to whether you are
pregnant or not.
Doctors will often use the quantitative test if they are closely monitoring the development of a
pregnancy. After implantation occurs, the hormone will begin to rise and should increase every 48-72 hours for the
next several weeks.
Progesterone: The follicle from which the egg was released is called the corpus
luteum. It will release progesterone that helps thicken and prepare the uterine lining for implantation. The corpus
luteum will produce progesterone for about 12-16 days (the luteal phase of your cycle.) When the egg is fertilized,
the corpus luteum will continue to produce progesterone for the developing pregnancy until the placenta takes over
around week 10. Progesterone is the hormone that helps maintain the pregnancy until birth. Sometimes, the failure
of the corpus luteum to adequately support the pregnancy with progesterone can result in an early pregnancy loss.
Progesterone inhibits immune responses, decreases prostaglandins, and prevents the onset of uterine