Normal First Trimester Exam
Page Links: Preconception, Chronology of Development, Ultrasound Findings, Early Gestational Sac, 5th Week, Decidua, Gestational Sac, Yolk Sac, Amnion and Chorion, MSD, Discriminatory hCG Levels, Mid-gut Herniation, Heart, Crown Rump Length (CRL), Nuchal Translucency (NT), International Standards for CRL and Gestational Age, Measurements and First Trimester Pregnancy Viability, References
Some commentary and all images courtesy of Jill Beithon RT, RDMS, RDCS, RVT
First Trimester Ultrasound Scan
Preceded by pre-test counseling, women should be offered a first trimester ultrasound (US) (11 to 13 6/7 weeks) according to ISUOG recommendations if there are clinical concerns, pathological symptoms, or specific indications. The full text of this article is available and provides details for the performance of the first trimester fetal ultrasound scan. 
Above left. Maternal ovary. Approximately 24 hours before ovulation, a hypoechoic ring within the GF is seen (cumulus oophorus), which contains the oocyte.
Above right. Maternal ovary. The corpus lutuem (CL) represents the ruptured GF, which is a thin-walled cyst with circumferential blood flow demonstrated by color Doppler. The CL secretes progesterone and a small amount of estrogen to stimulate endometrial proliferation.
Above. After fertilization  the zygote undergoes rapid cell division and 3 days later, just before entering the uterus, the 12 to 16 cell stage (morula) is present. Within the uterus, the conceptus is termed the blastocyst.
Above. The blastocyst is composed of an outer cell layer, the trophoblast, which is destined to form part of the placenta and an inner cell layer, which is destined to form the embryo, amnion, umbilical cord, and yolk sac.
Chronology of Development
Above. The embryo undergoes some of the following developmental sequences.  By the 8th week, the heart has a definite form and by the 10th week, the peripheral vascular system is complete. By 8 to 12 weeks, the mid-gut herniates into the base of the umbilical cord. By the 9th week, the 3rd and lateral ventricles are formed, and the fingers are distinct. By the 10th week, the anal membrane perforates. The kidneys ascend from the pelvis in the 8th week and are in their adult location by the 11th week.
Early Gestational Sac
Above. The earliest sign of an intrauterine pregnancy (IUP) is a small gestational sac seen on TVS at approximately 4 to 5 weeks and at that time, the mean sac diameter (MSD) is approximately 2.5 mm. The early sac should be round and adjacent to (not within) the linear interface of the endometrial lining, without displacing or deforming it.
This small round fluid collection (the chorionic cavity) is completely surrounded by a hyperechoic rim of tissue about 2 to 3 mm thick, which represents the developing chorionic villi and adjacent decidual tissue. The gestational sac is usually positioned in the mid to upper uterus.
Above. The gestational sac is now visible and contains the secondary yolk sac lying opposite the amniotic cavity with the embryonic disk (epiblast) between them. The epiblast will go on to form the definitive embryo and fetus. The extra-embryonic coelom is the subsequent chorionic cavity (the space between the amnion and chorion). By about 14 weeks, the chorionic cavity is normally obliterated and the amnion-chorion membrane forms the amniotic sac within which is the amniotic fluid (“bag of waters”).
Above. By 5 to 6 weeks, two concentric echogenic lines surround a portion of the gestational sac and can be seen on trans-abdominal ultrasound.
Above. On TVS, note the decidua capsularis and the decidua parietalis, findings which are sometimes referred to as the double decidual sign. Note the YS which is typically the first structure visualized in an IUP on TVS.
Above. On TVS, a gestational sac is identified at 5 weeks.  In patients with accurate dates, cardiac motion is seen typically after 5 5/7 weeks, while a yolk sac is first seen at 5 1/7ths to 5 5/7ths weeks.  When an embryonic heartbeat is first seen, the MSD is 20 mm and at the time of embryo movements, the MSD reaches 30mm. 
The yolk sac has an echogenic periphery with a sonolucent center. Detection of a yolk sac confirms that the intrauterine fluid represents an IUP and the number of yolk sacs usually correspond to the number of amnions. Yolk sac number is useful in assessing amnion number in multiple gestations. The diameter of the yolk sac increases between 5 to 10 weeks to a maximum of about 6mm,  and the measurement of the yolk sac is usually from inner wall to inner wall.
Amnion and Chorion
Above. The origin of the chorion is different from that of the amnion since the outer cell layer of the blastocyst forms the chorionic membranes and the inner cell layer forms the amnion. The chorion and amnion usually fuse by about 14 weeks. Typically, the amnion is visualized after the YS.
Above. The MSD is usually 5 mm at by approximately 5 weeks. The MSD is calculated by adding 3 orthogonal dimensions of the chorionic cavity, excluding the hyper-echoic rim of tissue, and dividing by 3 (Length + Width + Height/3). MSD growth in normal gestation increases approximately 1.1 mm/day, while the embryo grows at 1 mm/day. 
Discriminatory hCG Levels
The discriminatory β-hCG level is the value above which an intrauterine gestational sac is seen on a consistent basis by ultrasound. This value is commonly cited as between 1000 and 2000 mIU per milliliter (World Health Organization 3rd or 4th International Standard). As noted by Doubilet and others, a single hCG level does not reliably distinguish among a normal intrauterine pregnancy, an ectopic pregnancy, or a nonviable pregnancy.  (Please see below).
Above. Physiological herniation of bowel into the base of the umbilical cord is part of the normal developmental sequence which occurs at about the 6th to 8th weeks, and by 8 to 12 weeks, the mid-gut herniates back into the base of the umbilical cord.
Above. M-mode embryonic heart. Embryonic and/or fetal cardiac activity should be documented. Utilize M-mode – not pulsed Doppler – for that purpose. With regards to ultrasonic exposure settings, use the ALARA principle (As Low As Reasonably Achievable). At < 6 weeks, the normal fetal heart rate is 100-115 beats per min (BPM). A rate of < 90 BPM is considered embryonic bradycardia.
Crown Rump Length (CRL)
Above. Measured between 6 and 10 weeks, the CRL is the most accurate method for dating pregnancies. The longest axis of the embryo is measured and the yolk sac is identified separately from the embryo and is not included in the measurement.
Nuchal Translucency (NT)
Above. Normal NT.
Above. Increased NT.
The NT (See Genetic Markers) measures posterior neck nuchal fluid and a measurement of > 3 mm is always abnormal and is a marker for fetal aneuploidy and certain malformations, especially the heart.
The NT is measured at a gestation age of 11w0d to 13w6d weeks with a corresponding CRL of between 45 mm and 84 mm. A mid-sagittal view is taken in a neutral position away from the amnion. The maximum translucency is obtained with the callipers “on-to-on” as illustrated. A certification program is available through: The Fetal Medicine Foundation.
International Standards for CRL and Gestational Age
The following are charts from the open access article published relative to international standards for CRL in relationship to gestational age. 
The following chart is also from the open access article, which is freely downloadable. 
A calculator is available on: Perinatology.com. If the CRL is known, the estimated gestational age and expected nuchal translucency can be obtained. If the NT measurement is known, the calculator will yield the NT percentile.
Measurements and First Trimester Pregnancy Viability
Anatomy can be defined and some standard fetal measurements can be taken during the first trimester.
Errors exist during early first trimester in correctly assigning whether the pregnancy is viable, nonviable, or in an ectopic location. The false positive rate (incorrect diagnosis of a not viable pregnancy when the pregnancy is normal) is as high as 4.4%. 
In a recent review,  TVS diagnosis of pregnancy failure in a pregnancy with uncertain viability is as follows:
CRL of ≥ 7mm and no evidence of heartbeat.
MSD of ≥ 25 mm and no embryo visualized.
Initial scan shows a gestational sac without a yolk sac followed by a scan ≥ 2 weeks later demonstrating the absence of embryo with heartbeat.
Initial scan shows a gestational sac with a yolk sac followed by a scan ≥ 11 days later showing the absence of embryo with heartbeat.
If viability remains unclear, repeat the TVS in 11 to 14 days.
Abstract: PMID: 23280739 Moore, KL, Persaud TVN, Shiota, K. In Color Atlas of Clinical Embryology. WB Saunders Company; 1994; p.1. Rumack, CM, Wilson SR, Charboneau, JW. In: Diagnostic ultrasound. Mosby, St. Louis, 1998, p 977.
Abstract: PMID: 2030862
Abstract: PMID: 2660539
Abstract: PMID: 2030862 Rumack, CM, Wilson SR, Charboneau, JW. In: Diagnostic ultrasound. Mosby, St. Louis, 1998, p 997. Rumack, CM, Wilson SR, Charboneau, JW. In: Diagnostic ultrasound. Mosby, St. Louis, 1998, p 990.
Abstract: PMID: 24106937
Abstract: PMID: 25044000
Abstract: PMID: 25044000
Abstract: PMID: 21997898
Abstract: PMID: 24106937