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1、 前置胎盘前置胎盘 前置胎盘(placenta previa)是妊娠晚期出血的最常见的原因,是妊娠期的严重并发症,处理不当可危及母儿生命。定义孕28周后,若胎盘附着在子宫下段,其下缘达到或覆盖宫颈内口中,位置低于胎儿先露部,称为前置胎盘。发生率当受精卵在子宫体腔内低位着床时,很可能形成一个贴近宫颈内口的胎盘。如此附着的胎盘有三种结局:l 早期流产l 向宫底迁移:胎盘与子宫同步生长使得胎盘常被牵引向上进入宫体而离开宫颈l 留在原位,发展为前置胎盘。所以前置胎盘并不常见。发生率 国内0.241.57%;国外0.30.9%国内外统计结果均存在显著差异,反映对各种类型前置胎盘缺乏精确的定义及鉴定。难题
2、在于胎盘种植在子宫下段但其下缘与已扩张的宫颈有距离,局部发生剥离引起无痛性出血时,该归入前置胎盘抑或胎盘早期剥离,显然两种情况都存在。高危因素年龄:年龄越大,前置胎盘的发生率越高,40岁以上的孕妇其前置胎盘的发生率较20岁以下的高9倍。胎次与产次:多胎次与多产次的前置胎盘的发生率也增高。前置胎盘易发生于胎次4、产次3者。高危因素(续)自然流产、人工流产及前次剖宫产史:前两种情况除妊娠本身种植外,尚可能因刮宫使子宫内膜受损。凡有两次剖宫产史者本次发生前置胎盘的可能性增加。高危因素(续)吸烟、吸毒史:国外报道吸烟及嗜可卡因诱发前置胎盘。每日吸烟20支以上及嗜可卡因孕妇的前置胎盘发生率为无此嗜好孕妇
3、的1.42.0倍。吸烟孕妇的胎盘面积增大、重量增加,因为尼古丁可和促肾上腺皮质释放肾腺素,使血管收缩影响子宫胎盘血流量,而CO又致慢性血氧过少,胎盘为获取较多氧而肥大,即有可能覆盖宫颈内口。可卡因使血管收缩,妊娠期间可拮抗其作用的胆碱酯酶较少,在孕妇易感受可卡因引起的血管并发症。子宫血管发生痉挛,胎盘中的螺旋小动脉堵塞及毁坏,由此造成的灌注低下,刺激胎盘代偿性肥大,扩大面积以建立有效循环,胎盘前置的危险性因而增加。双胎妊娠等病因子宫内膜病变与损伤 多次刮宫、分娩、子宫手术史、产褥感染等,损伤子宫内膜,引起子宫内膜炎或萎缩性病变,再次受孕时子宫蜕膜血管形成不良,胎盘血供不足,刺激胎盘面积增大延伸
4、到子宫下段。手术瘢痕可妨碍胎盘在妊娠晚期向上迁移,易发生前置胎盘胎盘面积过大:多胎妊娠胎盘异常:如副胎盘、膜状胎盘受精卵滋养层发育迟缓临床分类完全性(或中央性)前置胎盘部分性前置胎盘边缘性前置胎盘低置胎盘:胎盘种植于子宫下段,其边缘接近宫颈内口 The traditional classification of placenta previa describes the degree to which the placenta encroaches upon the cervix in labour and is divided into low-lying,marginal,partial,
5、or complete placenta previa.In recent years,publications have described the diagnosis and outcome of placenta previa on the basis of localization,using transvaginal sonography(TVS)when the exact relationship of the placental edge to the internal cervical os can be accurately measured.The increased p
6、rognostic value of TVS diagnosis has rendered the imprecise terminology of the traditional classification obsolete.This guideline describes the current diagnosis and management of placenta previa and is based largely on studies using TVS.From SOGC 胎盘与宫颈内口的关系可随子宫下段的逐渐伸展、宫颈管的逐渐消失和宫颈口的逐渐扩张而改变。因此,前置胎盘的程
7、度可随妊娠、产程的进展而发生变化。临产前为完全性前置胎盘,临产后由于宫颈口的扩张,可变为部分性。所以,入院时的分类很可能与处理前的检查结果不一致,而以者决定其类型。临床表现症状l 突然、无痛、反复性的阴道出血(晚孕或临产时)l 贫血l 产后出血:由于子宫下段的蜕膜发育不良,前置胎盘可合并植入性胎盘,因而在子宫下段形成过程中及临产后不发生阴道出血,却在胎儿娩出后导致产后出血。体征:全身状况与出血量成正比诊断病史 妊娠晚期或临产后突然发生无痛性阴道流血,无任何诱因。以往有流产刮宫、产褥感染、剖宫产或子宫肌瘤剜出术史;与上次妊娠间隔不足6个月;高龄孕妇或多胎妊娠。本次妊娠中期产前检查时,B超示胎盘邻
8、近或覆盖宫颈内口。查体l 腹部检查:子宫软、轮廓清楚、无阵发性或强直性宫缩,其大小与长度符合孕周。胎位清楚,胎先露高浮或有骑跨现象(后壁胎盘)或其前方似有膨胀的膀胱(前壁胎盘)。胎心音清楚,一般无胎儿窘迫现象,除非母体已陷入休克状态。l 阴道检查:一般不用,除非必须通过阴道检查明确诊断或为终止妊娠决定分娩方式,则可在输液、备血或输血以及可立即手术的条件下进行。一般仅行阴道窥诊及阴道穹窿部扪诊,不作宫颈管内指诊,以防附着于宫颈内口处的胎盘进一步剥离引起大出血。必要时阴道检查的内容 严格消毒外阴、阴道后,先用窥阴器视有无阴道、宫颈局部病灶出血,如阴道壁静脉曲张破裂、宫颈息肉或糜烂出血等。继作阴道穹
9、窿部扪诊,以一手的示、中两指轻轻触摸宫颈周围的阴道穹窿部。若感觉在手指与胎先露之间有较厚的软组织,应考虑为前置胎盘;如清楚扪及胎先露,则可排除前置胎盘。一般不作宫颈管内指诊。要是发现宫颈口已扩张,则可将示指轻轻伸入宫颈。宫颈管内如有血凝块,触之易碎,但切忌用力触动,再触摸宫颈内口附近有无海绵样胎盘组织,并判断胎盘边缘与宫颈内口的关系,以确定前置胎盘的类型。若触及胎膜并决定终止妊娠,可刺破胎膜,羊水流出后,胎先露下降可压迫胎盘而减少出血或暂时止血。B超:一致被认为是最简单、最安全及最有价值的胎盘定位法。国内目前多用经腹部B超(TAS)l 早中期妊娠时,声像图上所显示的宫颈内口为解剖学内口,随妊娠
10、进展,子宫峡部扩展为子宫下段,解剖内口消失,此时所显示的宫颈内口为学内口。由于下段的形成变长和胎盘上半部的不断增长,使胎盘下缘与宫颈内口的距离逐渐拉大。因此,B超诊断前置时,须注意孕周。孕20周前,胎盘占据子宫壁一半面积,故而胎盘邻近或覆盖宫颈解剖学内口的机会较多,若发现胎盘位置低,宜诊断为“胎盘前置状态”,应定期随访至34周后再下结论。B超断层显像可显示子宫壁、宫颈、胎儿先露部和胎盘。显像屏上胎盘呈现为为轮廓清晰的半月形弥漫光点区。膀胱适当及充盈时,在耻骨联合上方作横切面扫描可显示胎盘下缘宫颈内口的前后位置关系,从而判断前置胎盘的类型。附着于子宫前壁的前置胎盘易于诊断,但若膀胱过度充盈,可将
11、前壁胎盘压向宫颈内口,造成胎盘低置或前置的假象。附着于子宫后壁的前置胎盘,由于子宫扩大,腹部探测深度不够或被胎儿先露部遮盖,往往不易显示而有可能漏诊。检查后壁胎盘时,可将胎儿先露部朝宫底方向轻推,或置孕妇于头低足高位,使胎儿先露部远离宫颈内口,易见胎盘下缘图像。另一漏诊原因为胎盘主体在子宫体部,以致疏忽了胎盘且向下扩展至宫颈内口的可能性。总体说来,经TAS定位准确率仅达95%左右。Transvaginal sonography,if available,may be used to investigate placental location at any time in pregnancy
12、when the placenta is thought to be low-lying.It is significantly more accurate than transabdominal sonography,and its safety is well established.(ll-2A)Sonographers are encouraged to report the actual distance from the placental edge to the internal cervical os at TVS,using standard terminology of m
13、illimetres away from the os or millimetres of overlap.A placental edge exactly reaching the internal os is described as 0 mm.When the placental edge reaches or overlaps the internal cervical os on TVS between 18 and 24 weeks gestation(incidence 24%),a follow-up examination for placental location in
14、the third trimester is recommended.Overlap of more than 15 mm is associated with an increased likelihood of placenta previa at term.(ll-2A).When the placental edge lies between 20 mm away from the internal os and 20 mm overlap after 26 weeks gestation,ultrasound should be repeated at regular interva
15、ls depending on the gestational age,distance from the internal os,and clinical features such as bleeding,because continued change in placental location is likely.Overlap of 20 mm or more at any time in the third trimester is highly predictive of the need for CS.(lll-B)产后检查胎盘及胎膜:注意胎盘形态,并注意有无副胎盘。若胎盘边缘
16、或部分胎盘有紫黑色陈旧凝血块附着,表明为胎盘的前置部分,诊断可以明确。如胎膜破口距该处胎盘边缘在7cm以内,为部分性、边缘性或低置胎盘的佐证。在行CS时,术中可直接了解胎盘位置,胎膜破口失去诊断意义。综上所述,在孕28周后,经B超检查、阴道检查、剖宫产或阴道分娩后确定胎盘附着部位异常者,方可诊断为前置胎盘。孕28周前属流产范畴。对母儿的影响对母体的影响:l 失血l 植入性胎盘(placenta accreta):子宫下段的蜕膜发育差l 羊水栓塞:附着处病理性开放的子宫静脉窦前置胎盘是发生羊水栓塞的诱因之一。l 产褥感染:创面位置低、失血贫血、手术Women with a placenta previa and a prior CS are at high risk for placenta accreta.If there is imaging evidence of pathological adherence of the placenta,delivery should be planned in an appropriate setting with adequate reso