EARLY FETAL DEVELOPMENT
The development of a baby is quite an intricate journey. From the moment that the egg and sperm
meet, a baby is beginning the developmental process. This early part of development lays the foundation for a
healthy pregnancy and the birth of a healthy baby. Unfortunately, because these early weeks involve such a complex
process, things can go wrong and ultimately end in a pregnancy loss. If a possible complication in early pregnancy
is suspected, your health care provider will use a combination of blood tests and ultrasound tests to make a clear
diagnosis. A blood test can be used to monitor hCG levels and progesterone levels. Ultrasounds can be used to
visually see what development is taking place in the uterus and to measure the progress.
It is common to have many questions about what this early development truly involves and what is
to be expected. We have gathered information from different sources in order to provide the best guidelines of what
normal early fetal development looks like. However, just as every woman is different, every pregnancy develops
differently. This information should be used as a general guide for healthy pregnancy development, although
development may vary due to the mother’s health or a miscalculation of ovulation. Gestational age is the age of the
pregnancy from the last normal menstrual period (LMP), and fetal age is the actual age of the growing baby. Most
references to pregnancy are usually in gestational age rather than fetal age development, but we have included both
so that it is clear what stage development is at.
Week 1 & 2 Gestational Age - (Conception)
At this stage, the menstrual period has just ended and your body is getting ready for ovulation.
For most women, ovulation takes place about 11 - 21 days from the first day of the last menstrual period. During
intercourse, several hundred million sperm are released in the vagina. Sperm will travel through the cervix and
into the fallopian tubes. When conception takes place, the sperm will penetrate an egg and create a single set of
46 chromosomes called a zygote - the basis for a new human being. The fertilized egg, called a morula, spends a
couple of days traveling through the fallopian tube toward the uterus and dividing into cells (this dividing
process is where many chromosomal abnormalities occur). The morula becomes a blastocyst and will eventually end up
in the uterus. Anywhere from day 6 - 12 after conception, the blastocyst will imbed into the uterine lining and
begin the embryonic stage.
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
Week 5 - Gestational Age (Fetal age 3 weeks)
The gestational sac is often the first thing that most transvaginal
ultrasounds can detect at about 5 weeks. This is seen before a recognizable embryo can be seen. Within this week,
at about week 5 ½ to the beginning of the 6th week, a yolk sac can be seen inside the gestational sac. The yolk sac
will be the earliest source of nutrients for the developing fetus.
Human chorionic gonadotropin (hCG) levels can have quite a bit of variance at this point. Anything
from 18 - 7,340 mIU/ml is considered normal at 5 weeks. Once the levels have reached at least 2000, some type of
development is expected to be seen in the uterus using high resolution vaginal ultrasound. If using a
transabdominal ultrasound, some type of development should be seen when the hCG level has reached 3600 mIU/ml.
Although development may be seen earlier, these levels provide a guide of when something is expected to be
Progesterone levels also can have quite a variance at this stage of pregnancy.
They can range from 9-47ng/ml in the first trimester, with an average of 12-20ng/ml in the first 5-6 weeks of
With both hCG levels and progesterone levels, it is not the single value that can predict a
healthy pregnancy outcome. It is more important to evaluate two different values to see if the numbers are
increasing. Levels of hCG should be increasing by at least 60 % every 2-3 days, but ideally doubling every 48-72
hours. Progesterone levels rise much differently than hCG levels, with an average of 1-3ng/ml every couple days
until they reach their peak for that trimester. In situations when there is a concern of an ectopic pregnancy or
miscarriage, hCG levels will often start out normal, but will not show a significant increase or will stop rising
all together, and progesterone levels will be low from the beginning.