•IVF-conceived pregnancy is increasingly common (1).

•Birthweight (BW) differs by mode of conception (MOC): spontaneously conceived (SC), fresh/frozen embryo transfer (ET) (2).

•Existing literature fails to account for confounding by prematurity.

•It is unknown whether IVF is associated with true aberrant fetal growth

•There is a gap in knowledge of how IVF-conceived fetuses grow in utero.

 

Hypothesis and Aims  

“In utero growth and birthweight differ by mode of conception”

This study collected pilot data to test feasibility and generate further research hypotheses

 

Materials and Methods  

A prospective IVF-conceiving cohort was recruited to START clinic study (REC: 20/NE/2022). A historic prospectively-studied SC control cohort was used as comparison (NUPS; REC: 10/H1013/9).

Aim 1: BW z-scores were compared between MOC by linear regression

Aim 2: serial ultrasound biometric parameters were measured (BPD, HC, AC, FL, TVol, AVol, 2D/3D EFW).

Ultrasound scan (USS)-determined gestational age was used for all pregnancies.

 

Serial 2D/3D USS measurements of fetal biometric parameters were plotted to map fetal growth by MOC.

  N=131 (SC N=103; fresh ET N= 9; FET N= 20).

All statistical analysis was carried out using Stata/BE (Stata Corp, Texas, USA). P value <0.05 was considered statistically significant and 95% Confidence Intervals (CI) were used.   

FET-conceived infants were born heavier for gestation than spontaneously conceived infants, even after adjusting for gestation at birth, maternal age and parity. This suggests that birth weight differences are not simply due to longer gestational ages

There was no suggestion of difference in skeletal growth between MOC type (P>0.1; data not shown). Differences in parameters influenced by soft tissue growth between SC and IVF groups were very subtle when graphed. Frozen ET pregnancies tended to lay down soft tissue faster than Fresh ET pregnancies. 

Frozen ET pregnancies demonstrate a subtly faster rate of EFW increase than SC and fresh ET pregnancies.

Discussion

Frozen ET conceived infants had higher BW (Table 1) compared to SC peers, even after accounting for gestational age, supporting current literature (3) and suggesting presence of potentially accelerated intrauterine growth.

Our results suggest potential differences in fetal soft tissue deposition may exist. The direction of such differences are inconsistent and limited by a lack of AVol data in the SC cohort.