Stage Four: Approximately six weeks after a pregnant woman's last period, we can see a small fetal pole, one of the first stages of growth for an embryo, which develops alongside the yolk sac.
When does the fetal pole appear? A fetal pole is often visible with vaginal ultrasound at about five and a half weeks of pregnancy. But sometimes it's not seen for several weeks, depending on the type of ultrasound and the angle of your uterus.
When is the yolk sac visible? The yolk sac begins to develop during the second week of gestation (pregnancy). A healthcare provider can see the yolk sac using transvaginal ultrasound starting at about week five. The yolk sac grows as pregnancy progresses from week five to week 10.
Week 4 - implantation
After some time, they will form the placenta (afterbirth). The inner group of cells develop into the embryo.
Intrauterine sacs embryonic structures
A yolk sac is usually seen by 6 menstrual weeks, or by the time the mean diameter of the sac has reached 10 mm (Fig 1B and Fig 1C). A fetal pole with heart tones is typically seen by the completion of 7 menstrual weeks (Fig 1D).
The lowest (“threshold”) β-hCG levels associated with visible pregnancy-related structures were 390 mIU/ml (gestational sac), 1094 mIU/ml (yolk sac), and 1394 mIU/ml (fetal pole).
The fetal pole is the first direct imaging manifestation of the fetus and is seen as a thickening on the margin of the yolk sac during early pregnancy. It is often used synonymously with the term "embryo".
Throughout the process of the first 12 weeks of gestation, the yolk sac goes through many changes. By week 12 of pregnancy, the embryo develops a placenta, and the placenta takes over for the yolk sac to provide nutrients and oxygen to the growing embryo.
When the HCG level reached 7200 mIU/ml, a yolk sac was seen in every patient. Ten of 22 patients with HCG between 1000 and 7200 mIU/ml had a visible yolk sac. Every patient with an HCG level greater than 10,800 mIU/ml had a visible embryo with a heartbeat.
The visualizing rate of the yolk sac in miscarriages after the embryo had been formed was significantly higher in those women who demonstrated fetal heart activity (82.1%) than in those who did not (54.5%). On the other hand, the yolk sac was observed in 44% of miscarriages without a visible embryo.
The yolk sac is a positive sign and an important indicator of pregnancy health. If you see it, it's reassurance that all is well and your little nugget is starting to develop as expected.
A blighted ovum, or anembryonic pregnancy, is when a fertilized egg implants in the uterine lining but does not grow into an embryo. The gestational sac and placenta will grow, but the gestational sac containing the embryo remains empty. It causes a miscarriage in the first trimester of pregnancy.
A blighted ovum, also called an anembryonic pregnancy, occurs when an early embryo never develops or stops developing, is resorbed and leaves an empty gestational sac. The reason this occurs is often unknown, but it may be due to chromosomal abnormalities in the fertilized egg.
If there are no signs of pregnancy or inconsistent signs, like a large gestational sac without any yolk sac or fetal pole, it may mean you have a blighted ovum or are otherwise miscarrying. This is very common in the earliest weeks of pregnancy, when the risk is the highest.
The fetal pole is a thickening on the margin of the yolk sac of a fetus during pregnancy. It is usually identified at six weeks with vaginal ultrasound and at six and a half weeks with abdominal ultrasound. However, it is not unheard of for the fetal pole to not be visible until about 9 weeks.
The yolk sac, visible from 35 days' gestation3, is the first structure to appear within the gestational sac, and indicates an intrauterine pregnancy with a positive predictive value of 100%4.
These organs include the yolk sac, amnion and allantois. The yolk sac supplies food material to the embryo. The amnion, by enclosing the embryo, provides protection.
In normal pregnancies, the yolk sac increases in size to about 11 weeks of gestation, after which it disappears to 12 weeks of gestation.
Stage Four: Approximately six weeks after a pregnant woman's last period, we can see a small fetal pole, one of the first stages of growth for an embryo, which develops alongside the yolk sac.
The fetal pole is the first direct imaging manifestation of the fetus and is seen as a thickening on the margin of the yolk sac during early pregnancy. It is often used synonymously with the term "embryo".
A fetal heartbeat may first be detected by a vaginal ultrasound as early as 5 1/2 to 6 weeks after gestation. That's when a fetal pole, the first visible sign of a developing embryo, can sometimes be seen. But between 6 1/2 to 7 weeks after gestation, a heartbeat can be better assessed.
At 5 weeks into pregnancy things are so small there is very little to see on ultrasound. Even at 6 weeks it can be difficult to see an embryo with some people.
It's too early
You may have counted the days incorrectly if you're unable to see the gestational sac. Something as simple as getting the dates wrong may be the reason why you don't see anything during a 5-week ultrasound. This is common and has everything to do with your human chorionic gonadotropin (hCG) levels.
The gestational sac encloses the developing baby and contains amniotic fluid. If it's not visible by around 5 weeks gestation, it may mean that the pregnancy is not viable or a miscarriage has already happened.