Cause:Idiopathic usually; seizure medication toxicity (1/100) only from carbamazepine (Nejm 1991;324:674); maternal Vit A ingestion 
10 000 IU q d esp in 1st trimester (Nejm 1995;333:1369); maternal auto-antibodies to folate receptors (Nejm 2004;350:134), may be the most common cause
10-25% births in US; similar in Southern China but in N China, rate is 6/1000 (Nejm 1999;341:1485) Risk incr × 2 w maternal obesity (Jama 1996;275:1089, 1093)
Si:Meningocele, myelomeningocele, etc
Lab:
Chem: 
-Fetoprotein, HCG, and estriol levels at 15-18 wk gestation from last menstrual period; if elevated, then get ultrasound; amniocentesis not needed if US ok (Nejm 1990;323:557); but maternal AFP elevations are associated w increased fetal loss, levels 2-3 × normal w a 2.5 × increase, levels >3 × w 10 × increases (Nejm 1991;324:662)
Xray:MRI imaging (J Neuroradiol 2004;31:3)
Rx:
Prevent w folic acid (Med Let 2004;46:17) >0.4 mg qd periconception, 0/2000 births vs 6/2000 (Jama 1995;274:1698; Nejm 1998;338:1060; 1992;327:1832 vs 1989;321:430); dramatically decreases neural tube defect risk (Nejm 1999;341:1485; Peds 1993;493:4); perhaps 4 mg po qd if pos family hx; now added to all US grain products, eg, flour (Nejm 1999;340:1449) and had decr overall incid by 20% (Jama 2001;285:2981) and 40% in Canada (Nejm 2007;357:135); mandatory fortification in 3rd world even more effective (Lancet 2006;367:1352)
Consider abortion
In utero repair better outcomes, less need for shunts but riskier than if done after delivery (Nejm 2011, 10.1056/NEJMoa1014379; Jama 1999;282:1819, 1826)
Delivery by c-section may result in less motor deficit? (Clin Obgyn 1998;41:393 vs Nejm 1991;324:662)