Results / Sixteen successful pregnancies yielded 92 fetuses

Results
Sixteen successful pregnancies yielded 92 fetuses.
Biometric measurements. At 9.5 DPC, all gestations appeared as rounded sacs with a diameter (mean + standard error) of 4.4 + 1 mm (Figure 1). Placentas and fetal poles were differentiable by 10.5 DPC (Figure 2). Correlation between CRL and GA was highly significant beyond 9.5 DPC (r = 0.97, P < 0.0001), and a simple regression equation was derived to predict GA: CRL/2 + 9 d (R² = 0.935, P < 0.001; Figures 3 and 4 and Table 1). BPD was a significant predictor of GA beyond 12.5 DPC (R² = 0.845, P < 0.001; Figures 3 and 5 and Table 2).

Study variables

Study variables. The study variables included both biometric measurements and evaluation of morphologic features. The biometric measurements assessed were gestational sac dimensions, crown-rump length (CRL), biparietal diameter (BPD), and thoracoabdominal diameter (TAD). The morphologic features evaluated included: presence of fetal heart activity, the 4 chambers of the heart, great vessels, axial and appendicular skeleton, thorax, abdomen, and brain.

Imaging

Imaging. Transabdominal US was carried out with a 15-MHz transducer (Acuson Sequoia System, 3.5 x 1 cm 15L8 transducer, Siemens Corporation, Mountain View, CA). The system we used costs between $300,000 and $400,000, depending on options and discounts. The transducer probe costs between $17,000 and $18,000.

Materials and Methods / Technique

We mated 36 female mice individually with 12 males in 3 cycles. Females were pretreated with hormonal supplementation with 10 IU pregnant mare serum gonadotrophin in normal
saline administered intraperitoneally 48 h prior to mating and 10 IU human chorionic gonadotrophin diluted in normal saline administered intraperitoneally immediately prior to mating. For each of 3 cycles, 12 different females and the same 12 males were paired overnight and separated the next morning. This practice allowed precise determination of coitus and was used in preference to presence or absence of vaginal plugs. Noon of the first day postcoitus was considered to be 0.5 d post-coitus (DPC), and nonpregnant animals were mated again in subsequent cycles. 

Ultrasound Diagnosis of Mouse Pregnancy and Gestational Staging - part 2

However, like magnetic resonance microscopy, the ultrasound backscatter microscopy system is technically complex and not readily available. In addition, various technical limitations, such as a narrow transducer aperture (5 mm) and only superficial depth penetration capability (7 mm), may limit broader applicability of this system. Although ultrasound backscatter microscopy appears to be highly valuable for rigorous anatomic and physiologic genotype-phenotype analyses, it may be cumbersome, if not impractical, for more simple tasks such as determining gestational age (GA).

Ultrasound Diagnosis of Mouse Pregnancy and Gestational Staging

Until recently, phenotypic analysis of mouse lines carrying natural or induced mutations has been limited by lack of accurate, nondestructive in utero imaging techniques. The need for such a tool has become increasingly important as molecular biologists become reliant upon mutagenesis in the laboratory mouse to understand the genetic underpinnings of human disease. For developmental biologists, the added burden of noninvasively imaging the phenotypic consequences of genetic alterations in embryonic mice has been a great limitation.