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.
Ifa possible complication in early fetal development is suspected, your health care providerwill use a combination of blood tests and ultrasound tests to makea clear diagnosis. A blood test can be used to monitor hCGlevels and progesterone levels. Ultrasounds can be used to visually see what development is taking place in the uterus and measure progress.
Because every women 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 ageof the pregnancy from the last normal menstrual period (LMP), and fetalage is the actual age of the growing baby.
Most references to pregnancyare usually in gestational age rather than fetal age development,but we have included both so that it is clear what stage developmentis being discussed.
Gestational Age Week 1 & 2 (Fetal Age: Conception)
At this stage, the menstrual period has just ended and your body is getting readyfor 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 vagin*. Sperm will travel through the cervix and into the fallopian tubes.
When conception takesplace, the sperm will penetrate an egg and create a single set of46 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 fallopiantube 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 vagin*l ultrasoundat this time will be the “decidual reaction,” which isthe thickening of the endometrium. The endometrium lining thickensas the blastocyst burrows into it. This cannot always be detected byultrasound—sometimes it may take a special eye or very goodequipment to see this “reaction” in the endometrium lining.
A key fact to remember when choosing an ultrasound is that a transvagin*lultrasound can detect development in the uterus about a week earlierthan a transabdominal ultrasound.
hCG, The Pregnancy Hormone
Once implantation occurs, the pregnancy hormoneHuman Chorionic Gonadotropin (hCG) will develop and begin to rise. This hormone will signalthat you are pregnant on a pregnancy test. hCG can be detected through two different types of blood tests, or through a urine test.
Aquantitative 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 closelymonitoring the development of a pregnancy. After implantation occurs,the hormone will begin to rise and should increase every 48-72 hoursfor the next several weeks.
Progesterone
The follicle from which the egg wasreleased is called the corpus luteum. It will release progesteronethat helps thicken and prepare the uterine lining for implantation. The corpus luteum will produce progesterone for about 12-16 days (theluteal phase of your cycle.)
When the egg is fertilized, the corpuslu*teum will continue to produce progesterone for the developing pregnancyuntil the placenta takes over around week 10. Progesterone is thehormone that helps maintain the pregnancy until birth.
Sometimes, thefailure of the corpus luteum to adequately support the pregnancy withprogesterone can result in an early pregnancy loss. Progesterone inhibitsimmune responses, decreases prostaglandins, and prevents the onsetof uterine contractions.
Gestational Age Week 5 (Fetal Age: Week 3)
Around 5 weeks, the gestational sac is often the first thing that most transvagin*lultrasounds can detect. This is seen before a recognizableembryo can be seen. Within this time period, a yolk sac can be seen inside the gestationalsac. The yolk sac will be the earliest source of nutrients for thedeveloping 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 vagin*l 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 varianceat this stage of pregnancy. They can range from 9-47ng/ml in the firsttrimester, with an average of 12-20ng/ml in the first 5-6 weeks ofpregnancy.
With both hCG levels and progesterone levels, it is not the singlevalue that can predict a healthy pregnancy outcome. It is more importantto 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, butideally doubling every 48-72 hours.
Progesterone levels rise much differently than hCG levels, with an average of 1-3mg/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 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 vagin*l ultrasound. The fetal pole is the first visible sign of a developing embryo. Thispole structure actually has some curve to it with the embryo’shead at one end and what looks like a tail at the other end.
The fetal pole now allows for a 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 2-4mm, and the heartbeat may be seen as a regular flutter when the CRL has reached 5mm.
If a vagin*l 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, from 6 ½ -7 weeks is the time when a heartbeatcan be detected and viability can be assessed. A normal heartbeatat 6-7 weeks would be 90-110 beats per minute. The presence of anembryonic heartbeat is an assuring sign of the health of the pregnancy.
Once a heartbeat is detected, the chance of the pregnancy continuingranges from 70-90% dependent on what type of ultrasound is used. Ifthe embryo is less than 5mm CRL, it is possible for it to be healthywithout showing a heartbeat, though a follow-up scan in 5-7 days shouldshow cardiac activity.
If your doctor is concerned about miscarriage,blighted ovum, or ectopicpregnancy, the gestational sac and fetal pole (if visible) willbe measured to determine what type of development should be seen. The guideline is that if the gestational sac measures >16-18mmwith no fetal pole or the fetal pole measures 5mm with no heartbeat(by vagin*l ultrasound), then a diagnosis of miscarriage or blightedovum 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 anabsenceof a fetal pole, then further testing should be done to rule out thepossibility 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 anadult human is present in the developing embryo. The embryo has reachedthe end of the embryonic stage and now enters the fetal stage. A strongfetal heartbeat should be detectable by ultrasound, with a heartbeatof 140-170 bpm by the 9th week.
If a strong heartbeat is not detectedat this point, another ultrasound scan may be done to verify the viabilityof the fetus.
If a pregnancy has been diagnosed as non-viable, mostphysicians will give the choice of waiting to see if the body willmiscarry naturally (pending no other health issues) or to have a Dilation& Curettage (D&C) procedure. About 50% of women do not undergoa D&C procedure when an early pregnancy loss has occurred.
The hCG levels will peak at about 8-12 weeks of pregnancy and thenwill decline, remaining at lower levels throughout the remainder ofthe pregnancy. If the levels are questionable, an ultrasound scanshould be used to diagnose the pregnancy outcome. Ultrasound findingsare much more accurate at diagnosing pregnancy viability after 5-6weeks 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:
- 1-28 ng/ml Mid Luteal Phase (Average is over 10 for un-medicatedcycles and over 15 with medication use)
- 9-47 ng/ml First trimester
- 17-146 ng/ml Second Trimester
- 49-300 ng/ml Third Trimester
*There are many averages for progesterone levels.These charts are a very broad guideline—speak with your healthcare professional for more specific guidelines for you
**Remember – These numbers are just a GUIDELINE — every woman’shormone level can rise differently. It is not necessarily the levelthat matters but rather the change in the level.
Want to Know More?
- Bonding With Your Baby: Making the Most of the First Six Weeks
- 7 Common Discomforts of Pregnancy
- Pregnancy Nutrition
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
