In addition, there is evidence that the association of BMI with PTB may vary by parity, with some suggestion of a stronger association of obesity with SPTB among nulliparous women [17, 18] and of different associations of underweight with SPTB and MPTB among nulliparous and parous women [19], although the numbers of women with underweight in these studies have been small. Here, SPTB is linked to obesity disorder.