**3. Human pregnancy and fetal development**

The human gestation is about 280 days and is divided into three trimesters, each of which is marked by specific fetal developments and embryonic changes. The first trimester is from gestation week (GW) 1–12, including the conception, second trimester is GW 13–28 and third trimester is GW 29–40. In other mammals, the gestation is defined as the time between conception/fertilization and birth, which for comparison is 266 days in humans. A single fetus is carried in 97–98% of all human pregnancies. The human newborn is extremely dependent on the mother and has immature motoric skills which traditionally placed the neurodevelopment of the human newborn as *altrical* (from Latin: "to nurse"), referring to the undeveloped motoric system. However, the advanced development of the human brain at birth rather places the human newborn as *precocial*, meaning well-developed at birth [2]. The human brain at birth is more advanced than all other animal models used in research [3].

The placenta is the interface between the maternal and fetal circulation, facilitating an exchange of oxygen, nutrients, waste products and other molecules, for example, certain drugs. The fetal trophoblast cells form the external component of the placenta, the *chorionic plate*. The nomenclature of the placenta barrier refers to the degree of erosion of the maternal tissue in the uterine cavity and the interface between the maternal and fetal circulation. The placental interface differs greatly between species (**Figure 1**). Humans have *hemomonochorial* placenta barrier because the maternal blood (*hemo-*) is in direct contact with only one layer of trophoblasts (*−mono*) in the chorion plate (*−chorial*). Thus, the human placenta is implanted completely within the uterus with a deep invasion of the trophoblasts and erosion of the uterine epithelium [4]. In rodents (mice and rats), the placenta barrier is *hemotrichorail* with three layers of trophoblasts dividing the maternal blood from the fetal capillaries in the chorionic plate [5]. Another nomenclature used for the microscopic structure of the placenta exchange area refers to the villous or labyrinth type. The human placenta is of the villous type where chorionic vessels branch out with few interconnections. In a placenta of the labyrinth type, the fetal vessels, the trophoblasts and the maternal blood space branch out and are interconnected in a complex labyrinthine pattern [5].

#### **3.1. Gestation length**

Research in the pregnant human is problematic and may pose significant ethical restrictions as the well-being of the mother and her unborn baby is critically important. Thus, the use of animal models provides a way to gain insight into the improved understanding of the human pregnancy. Animal models remain essential to understand the fundamental mechanisms underlying the onset of obstetric diseases, and to discover improved methods for prevention, diagnosis and treatment. However, the translatability between animals and humans should be carefully considered. In obstetric research, several factors may contribute in the selection of the most appropriate animal model, concerning the mother, fetus and placenta. In addition, the timing of the study during the gestation needs to be considered, as pregnancy is a

344 Experimental Animal Models of Human Diseases - An Effective Therapeutic Strategy

The aim of this chapter is to review the advantages and limitations of relevant animals, including mouse, rat, chinchilla, guinea pig, sheep and pig models, and their use in studying fetal growth disorders (intrauterine growth restriction, IUGR), preeclampsia and diabetes in pregnancy. Furthermore, available imaging modalities for studying pregnant animals including fetal and placental characteristics are presented. Finally, ethical and welfare considerations are described, as well as how physiological effects of pregnancy pose special

Knowledge of the different characteristics of the animal and its gestation is a prerequisite in order to select the most suitable animal model, interpret experimental findings and reach appropriate translational conclusions. In obstetric research, in particular, it is necessary to consider fetal/neonatal characteristics and the physiological changes during the gestation period. Several species have been used to study the normal pregnancy and related pathologi-

The human gestation is about 280 days and is divided into three trimesters, each of which is marked by specific fetal developments and embryonic changes. The first trimester is from gestation week (GW) 1–12, including the conception, second trimester is GW 13–28 and third trimester is GW 29–40. In other mammals, the gestation is defined as the time between conception/fertilization and birth, which for comparison is 266 days in humans. A single fetus is carried in 97–98% of all human pregnancies. The human newborn is extremely dependent on the mother and has immature motoric skills which traditionally placed the neurodevelopment of the human newborn as *altrical* (from Latin: "to nurse"), referring to the undeveloped motoric system. However, the advanced development of the human brain at birth rather places the human newborn as *precocial*, meaning well-developed at birth [2]. The human brain

requirements to the management of feeding, handling, care and anesthesia.

cal conditions [1]. First, the human gestation is described for comparison.

at birth is more advanced than all other animal models used in research [3].

**3. Human pregnancy and fetal development**

dynamic process.

**2. Considerations**

A short gestation time (or rapid reproduction) is sometimes considered an advantage to obtain a high experimental productivity or for economic reasons. However, if repeated procedures are required during the gestation time, a longer gestation period is usually preferred. A longer interval between the experiments allows for longer restitution and thereby, reduces the induced stress response in the animals. In addition, surgical manipulation might be difficult to employ in animals with a short gestation. During a long gestation time, the response of environmental or physiological influences on the fetal development could also become more pronounced.

#### **3.2. Number and size of fetuses**

Occurrence of a single fetus in uterus is obviously preferred for individual fetal monitoring. A small number of fetuses will often correlate with bigger fetal size [6]. A bigger fetus makes it possible to receive a higher spatial resolution and sensitivity using non-invasive diagnostic tools, for example, clinical magnetic resonance imaging (clinical MRI) and computerized axial tomography (CT or CAT). Furthermore, surgical procedures are easier to perform. However, larger litter sizes provide a higher sampling size per gestation, and thus, the number of animals used can be reduced in accordance with the "3 R's" (see Section 9).

#### **3.3. Placentation**

Many differences exist in relation to placentation in the different animal models, such as the development and changes of the placenta during the time of gestation, blood flow, transfer of oxygen, nutrients and waste products, metabolic, endocrine and immunologic function [4].

In relation to drug transfer, it seems obvious to use animal models to study the passage across placenta and potential teratogenic or toxic effects, but unfortunately the transplacental transfer and the placental metabolic demand vary greatly among species [7]. Drugs may transfer across placenta by passive transport, active transport or facilitated transport, whereas lipidsoluble molecules with a small molecular size can cross the placenta by passive diffusion. In that regards, the guinea pig seems as a more human translatable model compared to, for example, the sheep, because the guinea pig has a thin *hemomonochorial* placenta barrier compared to the thicker *epitheliochorial* placenta barrier in the sheep [7]. For hydrophilic molecules, the passive diffusion is negligible, and the transport capacity varies widely between species depending on the transport proteins located in the trophoblast cells. For example, the antidiabetic drug metformin, a hydrophilic molecule, is in humans transported across the placenta by organic cation transporters (OCTs). When studied in an animal model, it is highly relevant to identify the specific OCT transporters in the pregnant animal to verify the expres-

**Table 1** shows the average gestation length, number of fetuses, maternal weight, neonate

Maternal prepregnancy weight

20 5–6 19 1 Hemotrichorial

22 9 283 6 Hemotrichorial

266 1 5900 3183 Hemomonochorial villous

67 3–4 728 80 Hemomonochorial labyrinth

113 1–2 480 40 Hemomonochorial labyrinth

30 5 1591 39 Hemodichorial labyrinth

153 1–2 39.100 2376 Epitheliochorial

115 5–14\* 84.000\* 400–1900<sup>a</sup> Epitheliochorial

Neonate weight (g) Placenta barrier type

Animal Models of Fetal Medicine and Obstetrics http://dx.doi.org/10.5772/intechopen.74038 347

labyrinth

labyrinth

(g)

sion of the transport proteins [8]; otherwise the translatability has little value.

weight and the placental barrier type in human and relevant species.

Number of fetuses

**4. Animal models**

Animal species

*Rattus norvegicus*

Guinea pig *Cavia porcellus*

Chinchilla *Chinchilla lanigera*

Rabbit *Oryctolagus cuniculus*

Sheep *Ovis aries*

Pig *Sus scrofa*

*Latin*

Human *Homo sapiens*

Mouse *Mus musculus*

Rat

**Basal gestation parameters of the laboratory animals**

Gestation length (days)

Data are acquired from the PanTheria database [82].\* Dependent of the breed of pig (domestic pig or mini-pig) [20].

**Table 1.** Basal gestation parameters in pregnant animal models.

**Figure 1.** Schematic presentation of different placenta barriers as seen in a microscope. (A) Hemomonochorial placenta barrier as seen in, for example, human, guinea pigs and chinchillas. Only one layer of syncytiotrophoblasts separates the maternal blood space from the fetal capillaries. (B) Hemodichorial placentabarrier as seen in the rabbit. One layer of syncytiotrophoblasts and one layer of cytotrophoblasts separate the maternal blood space from the fetal capillaries. (C) Hemotrichorial placenta barrier as seen in, for example, mice and rats. Three layers of trophoblast cells separate the maternal blood space from the fetal capillaries. (D) Epitheliochorial placenta barrier as seen in, for example, sheep. One layer of uterine epithelium cells and one layer of trophoblast cells separate maternal and fetal capillaries. Furthermore, in all three cases, maternal and fetal blood is separated by connective tissue and basal laminae.

In relation to drug transfer, it seems obvious to use animal models to study the passage across placenta and potential teratogenic or toxic effects, but unfortunately the transplacental transfer and the placental metabolic demand vary greatly among species [7]. Drugs may transfer across placenta by passive transport, active transport or facilitated transport, whereas lipidsoluble molecules with a small molecular size can cross the placenta by passive diffusion. In that regards, the guinea pig seems as a more human translatable model compared to, for example, the sheep, because the guinea pig has a thin *hemomonochorial* placenta barrier compared to the thicker *epitheliochorial* placenta barrier in the sheep [7]. For hydrophilic molecules, the passive diffusion is negligible, and the transport capacity varies widely between species depending on the transport proteins located in the trophoblast cells. For example, the antidiabetic drug metformin, a hydrophilic molecule, is in humans transported across the placenta by organic cation transporters (OCTs). When studied in an animal model, it is highly relevant to identify the specific OCT transporters in the pregnant animal to verify the expression of the transport proteins [8]; otherwise the translatability has little value.
