Early Fetal Development

Fetal Development

The development of a baby is a beautifully intricate process. From the moment the egg and sperm meet, your baby is growing. This early part of development lays the foundation for a healthy pregnancy and delivery. It is important to be informed in order to address any concerns regarding early fetal development.

If a possible complication in early fetal development 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 measure progress.

Because every woman is different and every pregnancy develops differently, this information should be used as a general guide for healthy pregnancy development, although early fetal 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 of development is being discussed.

Gestational Age Week 1 & 2 (Fetal 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 sperms are released into 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 embed into the uterine lining and begin the embryonic stage.

Gestational Age Weeks 3-4 (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 choosing an ultrasound is that a transvaginal ultrasound can detect development in the uterus about a week earlier than a transabdominal ultrasound.

hCG, The Pregnancy Hormone

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 simply detects the presence of hCG.

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 contractions.

Gestational Age Week 5 (Fetal Age: Week 3)

Around 5 weeks, the gestational sac is often the first thing that most transvaginal ultrasounds can detect. This is seen before a recognizable embryo can be seen. Within this time period, 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 2,000, some type of development is expected to be seen in the uterus using high-resolution vaginal ultrasound.

If a transabdominal ultrasound is used, 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 seen.

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 12-20 ng/ml in the first 5-6 weeks of pregnancy.

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 1-3 mg/ml every couple of 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 altogether, and progesterone levels will be low from the beginning.

Gestational Age Week 6 (Fetal age: 4 weeks)

Between 5 ½ to 6 ½ weeks, a fetal pole or even a fetal heartbeat may be detected by vaginal ultrasound.  The fetal pole is the first visible sign of a developing embryo. This pole structure actually has some curve to it with the embryo’s head at one end and what looks like a tail at the other end.

The fetal pole now allows for crown-to-rump measurements (CRL) to be taken so that pregnancy dating can be a bit more accurate. The fetal pole may be seen at a crown-rump length (CRL) of 2-4 mm, and the heartbeat may be seen as a regular flutter when the CRL has reached 5mm.

If a vaginal ultrasound is done and no fetal pole or cardiac activity is seen, another ultrasound scan should be done in 3-7 days. Due to the fact that pregnancy dating can be wrong, it would be much too early at this point to make a clear diagnosis of the outcome of the pregnancy.

Gestational Age Week 7 (Fetal Age: 5 weeks)

Generally, 6 ½ – 7 weeks is the time when a heartbeat can be detected and viability can be assessed. A normal heartbeat at 6-7 weeks would be 90-110 beats per minute. The presence of an embryonic heartbeat is an assuring sign of the health of the pregnancy.

Once a heartbeat is detected, the chance of the pregnancy continuing ranges from 70 – 90% depending on what type of ultrasound is used. If the embryo is less than 5 mm CRL, it is possible for it to be healthy without showing a heartbeat, though a follow-up scan in 5-7 days should show cardiac activity.

If your doctor is concerned about miscarriage, blighted ovum, or ectopic pregnancy, the gestational sac and fetal pole (if visible) will be measured to determine what type of development should be seen.  The guideline is that if the gestational sac measures >16-18mm with no fetal pole or the fetal pole measures 5mm with no heartbeat (by vaginal ultrasound), then a diagnosis of miscarriage or blighted ovum is made.

If the fetal pole is too small to take an accurate measurement, then a repeat scan should be done in 3-5 days. If there is an absence of a fetal pole, then further testing should be done to rule out the possibility of an ectopic pregnancy.

Gestational Age Week 8 & 9 (Fetal Age: 6-7 weeks)

By this point in the pregnancy, everything that is present in an adult human is present in the developing embryo. The embryo has reached the end of the embryonic stage and now enters the fetal stage. A strong fetal heartbeat should be detectable by ultrasound, with a heartbeat of 140-170 bpm by the 9th week.

If a strong heartbeat is not detected at this point, another ultrasound scan may be done to verify the viability of the fetus.

If a pregnancy has been diagnosed as non-viable, most physicians will give the choice of waiting to see if the body will miscarry naturally (pending no other health issues) or to have a Dilation & Curettage (D&C) procedure. About 50% of women do not undergo a D&C procedure when an early pregnancy loss has occurred.

The hCG levels will peak at about 8-12 weeks of pregnancy and then will decline, remaining at lower levels throughout the remainder of the pregnancy. If the levels are questionable, an ultrasound scan should be used to diagnose the pregnancy outcome. Ultrasound findings are much more accurate at diagnosing pregnancy viability after 5-6 weeks gestation than hCG levels are.

Guideline to hCG levels during pregnancy:

hCG levels in weeks from LMP (gestational age)* :

  • 3 weeks LMP: 5 – 50 mIU/ml
  • 4 weeks LMP: 5 – 426 mIU/ml
  • 5 weeks LMP: 18 – 7,340 mIU/ml
  • 6 weeks LMP: 1,080 – 56,500 mIU/ml
  • 7 – 8 weeks LMP: 7,650 – 229,000 mIU/ml
  • 9 – 12 weeks LMP: 25,700 – 288,000 mIU/ml
  • 13 – 16 weeks LMP: 13,300 – 254,000 mIU/ml
  • 17 – 24 weeks LMP: 4,060 – 165,400 mIU/ml
  • 25 – 40 weeks LMP: 3,640 – 117,000 mIU/ml
  • Non-pregnant females:
  • Postmenopausal:

Guideline to Progesterone Levels During Pregnancy:

*There are many averages for progesterone levels. These charts are a very broad guideline—speak with your health care professional for more specific guidelines for you
**Remember – These numbers are just a GUIDELINE — every woman’s hormone level can rise differently. It is not necessarily the level that matters but rather the change in the level.

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Compiled using information from the following sources:

1. Current Obstetric & Gynecologic Diagnoses & Treatment, Ninth Ed., DeCherney, Alan H., et al, Ch 8, 14

2. Williams Obstetrics Twenty-Second Ed. Cunningham, F Gary, et al, Ch 3

3. eMedicine

4. Creighton University Medical Center