When, within the female cycle, does conception occur in spontaneously fertile cycles? This study provides reference values of day-specific probabilities of date of conception in ongoing pregnancies. The maximum probability of being within a 5-day fertile window was reached on Day 12 following the last menstrual period (LMP). The true date of conception is not observable and may only be estimated. Accuracy of these estimates impacts on obstetric management of ongoing pregnancies. Timing of ovulation and fertility has been extensively studied in prospective studies of non-pregnant fertile women using error-prone proxies, such as hormonal changes, body-basal temperature and ultrasound, yielding day-specific probabilities of conception and fertile windows. In pregnant women, date of conception may be retrospectively estimated from early pregnancy fetal measurement by ultrasound. Retrospective analysis of consecutive pregnancies in women referred for routine first-trimester screening, over a 3-year period (2009–2011) in a single ultrasound center (n = 6323). Within the overall population, 5830 cases with a certain date of last menses were selected for analysis. The date of conception was estimated using a crown-rump length biometry and an equation derived from IVF/ICSI pregnancies. Day-specific probabilities of conception were estimated across several covariates, including age, cycle characteristics and ethnicity, using deconvolution methods to account for measurement error. Overall, the day-specific probability of conception sharply rises at 7 days after the LMP, reaching its maximum at 15 days and returning to zero by 25 days. Older women tend to conceive earlier within their cycle, as did women with regular cycles and white and black women compared with Asian ethnicity. The probability of being within the fertile window was 2% probability at Day 4, a maximum probability of 58% at Day 12 and a 5% probability by Day 21 of the cycle. Although conception is believed to occur within hours following ovulation, a discrepancy is theoretically possible. However, when comparing our results to those of prospective studies, no such difference was found. The equation used for estimating the date of pregnancy was estimated in IVF/ICSI pregnancies, which could lead to potential bias in spontaneous pregnancies. However, in our population, the observed bias was negligible. Non-fertile cycles and early pregnancy losses are necessarily overlooked because of the nature of our data. Because of the wider access to retrospective data and the potential bias in prospective studies of ovulation monitoring, this study should broaden the perspectives of future epidemiologic research in fertility and pregnancy monitoring. None.

Objective: To describe the incidence and nature of prenatal brain damage following fetoscopic laser selective coagulation (FLSC) of placental vessels for twin‐to‐twin transfusion syndrome (TTTS).Design: Retrospective observational study.Setting: Single center cohort.Population: All consecutive cases referred for TTTS treated by FLSC between 2003 and 2015.Methods: After the FLSC, patients were followed weekly by ultrasound. Fetal magnetic resonance imaging (MRI) scans were systematically planned at 30–32 weeks of gestation.Main outcome measures: Brain damage diagnosed prenatally by ultrasound or MRI.Results: In total, 1023 cases were reviewed. Brain damage was diagnosed prenatally in 22/1023 (2.1%) cases. Diagnosis was performed by ultrasound prior to MRI in 18 (82%) cases. All lesions were within the spectrum of ischaemic haemorrhagic lesions. Postoperative twin anaemia polycythaemia sequence and recurrence of TTTS were significantly associated with brain damage.Conclusion: The incidence of prenatal brain damage is low following FSLC, and is strongly associated with incomplete surgery.Tweetable Following FSLC for TTTS, prenatal brain damage occurs in 2% of cases and is associated with incomplete surgery.