the preferred embodiment of the apparatus is shown in fig1 . apparatus 100 includes a sensor 103 and an electrical cable 101 with a connector 109 . the electrical cable has a stiffer part 102 adjacent to the sensor . the stiffer part of the cable is rigid enough to be guided unsupported through the vagina and cervix and flexible enough to yield when the sensor encounters an obstruction such as the uterine wall . in the preferred embodiment , a part of the cable is stiffened by enclosing it in a plastic tube . for example , the cable can be surrounded by shrink - tubing of an inner diameter of 0 . 25 cm and an outer diameter of 0 . 41 cm . to allow repositioning , this tube must be long enough , preferably 30 cm , to extend from the vagina when the sensor is in the preferred region . the sensor 103 has one or more emitters ( the light source ( s )) 104 and detectors ( the light detector ( s )) 105 . the preferred embodiment has two emitters and a detector on one side , the active side , as shown in fig2 . in the preferred embodiment , the other or inactive side of the sensor has at least one tactile marking such as a bump or an indentation 120 , as shown in fig3 and 4 . this marking helps the user properly orient the apparatus while inserting it into the vagina and through the cervix . the leading edge 110 of the sensor 103 is bevelled , as shown in fig4 to facilitate introduction later in labor through a cervix that may be well - applied to ( snug against ) the fetal presenting part 204 . the preferred embodiment of the cable 100 has a series of regularly spaced markings 108 . these markings provide a visual indication of the insertion depth of the sensor in the mother &# 39 ; s vagina . in addition , a ridge 107 is formed on the cable at a predetermined distance from the leading edge 110 of the sensor 103 . the sensor is introduced until the ridge 107 is at the sagittal suture of the fetal head . the sensor is then in the preferred region 205 . the method of using the apparatus is as follows . the user determines the location of the fetal back and the height and orientation of the fetal head by abdominal examination . the user then makes a vaginal assessment of cervical status using the bishop score . this score grades the cervix on five elements : dilatation , effacement , position , station ( of the fetal head : above , below , or at the ischial spine ), and consistency ( firm , soft , etc .). the vaginal examination also may precisely confirm the position of the fetal head . with the examining fingers 201 already in the vagina 202 and at the posterior cervix 203 , the user grasps the apparatus 100 by the stiff part of the cable 102 with the other hand 200 . the sensor 103 is then inserted into the vagina with the active side up . it is then threaded up between the index and middle fingers 201 of the examining hand . the finger - tips of that hand feel for the bump or indentation 120 on the inactive side of the sensor 103 . see fig5 , and 7 . the fingers 201 of the examining hand stretch the posterior cervix 203 to make room for the sensor . the user further advances the sensor into the uterus past the presenting part and past the transcervical region . the sensor is then in the preferred region . for a fetus at term , the ridge 107 will be flush with the presenting part 204 when the sensor is in the preferred region , as in fig6 . the user then removes his or her fingers from the vagina , leaving the sensor 100 in place , as in fig7 . if the readings from the initial placement are unsatisfactory , the sensor can be repositioned by slightly withdrawing or further inserting the stiff part of the cable 102 . the preferred method of constructing the cable &# 39 ; s features is as follows . plastic tubing , such as polyolefin made by raychem , is heat - shrunk over the area of the cable to become the stiff part . this first layer stiffens the cable . regularly spaced markings are then drawn on the plastic tubing . in the preferred embodiment the markings are 1 cm apart , although any convenient marking scale can be used . the markings can take various forms , such as colors , characters , or numbers . the ridge is formed by heat - shrinking a short segment of plastic tubing around this cable assembly at an appropriate location . in the preferred embodiment , the ridge is 15 cm from the leading edge of the sensor . that is the approximate distance from the vertex of a molded fetal head at term to the preferred region for the sensor . the ridge may be used as a visual marking as well by making it a different color from that of the cable . after the ridge and the visual markings are applied to the cable , a clear plastic sheath is heat - shrunk over the cable assembly . the sheath smoothes the edges of the ridge and protects the visual markings . it also further stiffens the cable .
an intrauterine pulse oximetry apparatus and method are provided to measure fetal blood oxygen saturation at sites in a preferred region on the fetus . the sensor of the apparatus is attached to a stiff cable which can be inserted into the uterus past the presenting part and transcervical region of the fetus . the cable may have visual and tactile markings by which the user , without seeing or feeling the sensor , can guide it to an appropriate site on the fetus .