* Haifa, Israel;
** Femicare, s.r.o., Center of prenatal ultrasonographic diagnostics, Martin, Slovak republic. UVN SNP Ruzomberok, Gynecological and obstetrical department. Catholic University in Ruzomberok, Faculty of Health Care, Ruzomberok, Slovak Republic.

Introduction

The intervillous space (drawing 1) is the area between the chorionic plate and the basal plate of the placenta. Eroded uterine spiral arteries empty into the intervillous space and wash the chorionic villi that arise from chorionic plate and richly branch within the space. The villi contain capillaries that are terminal parts of fetal umbilical vessels. Maternal blood, which enters the intervillous space via the eroded spiral arteries, washes the chorionic villi and gets back to the maternal venous system via eroded uterine venules that also open into the intervillous space of the placenta.

Drawing 1: Intervillous space of the placenta – simplified drawing.

Maternal arteries and veins directly enter the intervillous space after 8th week of gestation, and at term, the intervillous space will contain about 150 ml of blood which is replenish about 3 or 4 times per minute [1]. Large amount of this pool of blood returns to mother’s circulation during labor by uterine contractions; thus, a woman undergoing cesarean section is liable to lose more blood than a woman having vaginal delivery.

The trophoblast, which is a collection of cells that invades the maternal endometrium to gain access to nutrition for the fetus, proliferates rapidly and forms a network of branching processes and invading and destroying the maternal tissues. This physiologic destructive process leads to opening of the maternal blood vessels (spiral arteries) of the endometrium. The vessels fill the spaces around trophoblastic network by maternal blood. The spaces around the trophoblastic network freely communicate and become greatly distended forming the intervillous space from which the fetus gains nutrition [1, 2].

The pressure in the spiral arteries is about 70 mm Hg, but falls to only 10 mm Hg in the intervillous spaces. The blood in the intervillous space is exchanged about 2-3 times per minute.

Endovascular and interstitial invasion are associated with the physiological conversion of the spiral arteries. During this process the arteries loose the smooth muscle in their walls and their elastic lamina and change into flaccid conduits (funnel shaped arteries). The extent of the conversion varies across the placental bed, and is greatest in the central region where trophoblast invasion is most extensive [3].

There are several theories trying to explain the mechanism of dynamics of the intervillous blood exchange throughout different stages of pregnancy.

In our understanding, based on transvaginal ultrasound examinations of fetuses between weeks 14-16, the placenta in a simplified way resembles a condensed “cauliflower-like structure” (chorionic villi) dipped in a “bowl” (basal plate). The "bowl" (basal plate) is penetrated be tiny holes representing openings of maternal vessels - spiral arteries (inflow) and veins (outflow).

In majority of cases the placenta has a homogeneous echogenic texture with discernible flow patterns at the level of its chorionic fetal plate and also within its decidual region (see images 1, 2 and video 1).

Images 1, 2, and video 1: The images and video show normal placental structure. In majority of cases the placenta has a homogeneous echogenic texture with discernible flow patterns at the level of its chorionic fetal plate (white arrow on the image 2) and also within its decidual region (yellow arrow on the image 2).

In 30% of the cases it is possible to visualize intra placental grooves and application of color Doppler on these regions reveals a laminar form of flow (see images 3, 4, 5 and videos 2, and 3).

Images 3, 4, 5, and videos 2, and 3: The images and videos show gray scale (image 3 and video 2) and color Doppler (images 4, 5 and video 3) scans of placenta with groows (arrows on the image 3) representing intervillous spaces with detectable laminar flow within the spaces (image 4, 5 and video 3).

Case 2 - linear or venous over-perfusion of the placenta

This was a low risk first pregnancy seen at 16 weeks. A cavernous structure of the placenta with linear flow within the caverns was present (see video 5). The outcome of the pregnancy was normal.

Image 7, video 5: 16 weeks of pregnancy. The image and video show cavernous structure of the placenta with linear flow within the caverns. The outcome of the pregnancy was normal.

Case 3 - arterial pulsating over-perfusion of the placenta

This was a low risk pregnancy seen at 15 weeks of gestation. The placenta showed enlarged lacunar space with two moderate systolic jets directed from basal plate into the enlarged lacune. The jets were making a whirlpool flow pattern within lacune. The findings were the same at 20 weeks of gestation. The outcome of the pregnancy was normal.

Images 8, 9, and videos 6, 7: 15 weeks of gestation - the images and video show enlarged lacunar space with two moderate systolic jets directed from basal plate into the enlarged lacune (video 6). The jets make a whirlpool flow pattern within lacune (image 9 and video 7). The findings were the same at 20 weeks of gestation and  the outcome of the pregnancy was normal.

Case 4 - arterial pulsating over-perfusion of the placenta

This was a low risk second pregnancy seen at 15 weeks. The placenta showed a huge maternal jet into a dilated placental lacune making a whirlpool flow within the lacune. At 17 weeks the finding was even more striking with another pulsating intra-lacunar vessel. The fetal growth, amniotic fluid and uterine arteries flows were normal. The outcome of the pregnancy was also normal.

Images 10, 11, 12, and video 8: 15 weeks of pregnancy; the images and video show a placenta with a huge maternal jet (image 12 and video 8) directed into a dilated placental lacune making a whirlpool flow within the lacune. At 17 weeks the finding was even more striking with another pulsating intra-lacunar vessel. The fetal growth, amniotic fluid and uterine arteries flows were normal. The outcome of the pregnancy was also normal.

Discussion

According to our experience the pulsating type caused by a huger arterial drainage into intervillous space has usually favorable prognosis. In literature a case of massive intraamniotic bleeding was described by Gilboa et al. [4]. To our believe a massive arterial or venous  filling of the intervillous space may, in some circumstances, lead to dramatic increase of intervillous pressure and consequent rupture of basal plate of the placenta, which was probably the cause of bleeding described by Gilboa et al.

1. Laminar type which may be secondary to three possible mechanisms
   • Overproduction of spiral arteries
   • Lower number of veins draining the intervillous space
   • Presence of a kind of venous fistula in the basal plate where venous pressure in a large vein is higher than the pressure within the intervillous space (10 mm Hg)
2. Pulsating type - caused by a larger artery draining directly to the intervillous space.

References

1.Sadler TW. Langman's Medical Embryology. 7th ed. Philadelphia: Lippincott Williams &Wilkins. 1995.

2.Rodney A. Rhoads David R.Bell:Physiology Third edition  Lippincott Williams & Wilkins ;2012

3.Yuping W. Shuang Z: Vascular Biology of Placenta :San Rafel:Morgan &Clapool Life Sciences;2010.

4.Gilboa Y, Duvdevani N, Yinon Y, Achiron R: A case of spontaneous intra-amniotic hemorrhage in an asymptomatic patient at near term pregnancy: Fetal Diagn Ther. 2012;31(1):73-5.