Embryonic/fetal growth following suckling-induced delay of implantation

In rats fertilized during the first or second day post partum (second consecutive pregnancy), suckling induces delay of implantation for 8 to 22 days. However, pregnancy is prolonged for only 3 to 17 days because accelerated embryonic and early fetal growth makes up 4 to 5 days of the implantation delay. After implantation, embryonic/fetal growth is accomplished within 11 to 12 days for a second consecutive pregnancy, whereas 16 days are required for first or second-spaced pregnancies. After weaning, increased function of the intestinal tract and liver is not needed anymore for mammary milk synthesis, and abundant nutrients can be shifted to the uteroplacental unit for rapid embryonic/fetal growth. Because the exponential curves for fetal growth are similar for first, second consecutive, and second-spaced pregnancies, it seems that, besides an increased supply of nutrients, an as yet unidentified maternal or a placentofetal factor(s) may play a role for embryonic/fetal growth.