Table 2 Number of events, risk difference, and numbers needed to treat (NNT) for adverse obstetric and neonatal outcomes according to discrepancy between pregnancy dating methods.

	Reference (n = 517657)		Large negative discrepancy (n = 119 275)				Large positive discrepancy (n = 113 236)
	Number of events	Event rate n/100 000	Number of events	Event rate n/100 000	Risk difference	NNT	Number of events	Event rate n/100 000	Risk difference	NNT
Pregnancy or delivery outcomes
Preeclampsia	16752	3236	4706	3946	0,0071	141	3564	3147	−0,0009	—
Diabetes mellitus in pregnancy	7091	1370	2374	1990	0,0062	161	1332	1176	−0,0019	—
Prolonged second stage of labor	3781	730	788	661	−0,0007	—	918	811	0,0008	1245
Delivery by forceps or vacuum extractor*	34412	6648	6996	5865	−0,0078	—	8324	7351	0,0070	142
Delivery by cesarean section	70122	13546	15019	12592	−0,0095	—	15370	13573	0,0003	3651
Delivery by emergency cesarean section**	35421	6843	7784	6526	−0,0032	—	7901	6977	0,0013	741
Shoulder dystocia*	1021	197	263	220	0,0002	4298	260	230	0,0003	3089
Postpartum hemorrhage	29413	5682	6274	5260	−0,0042	—	7226	6381	0,0070	143
Perineal laceration: third or fourth degree*	16340	3157	3220	2700	−0,0046	—	4014	3545	0,0039	258
Neonatal outcomes
Apgar score < 7 at 5 minutes	2816	544	825	692	0,0015	677	537	474	−0,0007
Birth asphyxia	4941	954	1340	1123	0,0017	592	1154	1019	0,0006	1548
Intrauterine fetal death	1314	254	446	374	0,0012	833	255	225	−0,0003
Neonatal death (<28 days)	624	121	314	263	0,0014	701	136	120	0,0000	—
Small for gestational age (<2 SDs)	10347	1999	3632	3045	0,0105	96	1873	1654	−0,0034
Large for gestational age (>2 SDs)	18320	3539	4619	3873	0,0033	300	4087	3609	0,0007	1423

Number of events, risk difference, and numbers needed to treat (NNT = 1/risk difference) for adverse obstetric and neonatal outcomes among all recorded singleton births in Sweden 1995–2010, with documentation of the date of last menstrual period, US-based estimated date of delivery, maternal weight, and height according to the discrepancy between pregnancy dating methods. Negative discrepancy indicates that the EDD by LMP was at an earlier date than was the EDD by US. Positive discrepancy indicates that the EDD by LMP was at a later date. A large negative discrepancy was defined as below the 10^th percentile, and a large positive discrepancy as above the 90^th percentile in the discrepancy distribution. The reference category (n = 517 657) was a discrepancy within 2 days of the median. The intermediate groups are not included in this comparison. NNT should in this context be interpreted as number needed to follow up more closely to possibly detect an adverse outcome. When NNT <1, the adverse outcome is expected to be less prevalent than in the reference category, and these numbers were omitted from the table.
*Excluding deliveries by cesarean section; **Excluding elective cesarean section.

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